As already stated, a Green Cheek rubbing their beak on you can mean several different things, it's what else they are doing at the same time as they are rubbing their beak on you that will tell you exactly what he is doing (sounds like a male from your description of your bird's behavior).
Obviously if he has just eaten something and he rubs his beak on you this is just him cleaning his beak off. No big deal. Sometimes if they are given a new perch, blanket, hammock or hut, toy, etc. they will rub their beak on the new surface (this is more of a dragging motion, they drag their beak back towards themselves). This is just them checking out the texture of the new surface, they are very inquisitive birds. Again, no big deal.
Now, what you describe with your bird head bobbing combined with rubbing his beak on you is most likely a hormonal behavior. He may also soon try to regurgitate for you after rubbing his beak on you and head bobbing. He's displaying his desire to mate with you. Birds (particularly male birds) that are trying to mate with a female bird inside a cage on a perch, or out in the wild on a tree branch, will continuously tap and rub their beaks back and forth on the perch or branch in front of the female bird. They combine this with head bobbing, dancing, singing/chirping/talking, preening the female, and finally regurgitating and feeding the female. This is typically the final step before they mate, if the female is willing. If she is not then she will not preen him back, nor will she accept his feeding. He'll continue to try to sway her until she either gives in (usually doesn't happen if the female isn't interested), disappears, or another female ones along that shows interest in him and that he is also interested in.
Birds are very much like humans in their courting and mating behaviors, they aren't like most other wild animals that you can just put a male and female of breeding age that are in season together and they'll mate, like dogs or cats. Birds court, try to impress their desired mates, reject mates they don't like or want, have mates that become just friends but not sexual mates that breed, etc.
It's important that you don't encourage this behavior in your Nanday, as it will not only make the behavior continue, but he will try to escalate the behavior and then when you don't mate with him in the end he can become physically and mentally frustrated, feel rejected by you, his chosen mate, and unfortunately he can also become very aggressive.
You need to show him that you love him and that he's a part of your family, and show him that you want to bond with him, but in a strictly platonic way. You need to try to knock him out of "breeding season" or "breeding mode", and once that happens and his hormones stop raging, this behavior will slow and stop as long as you continue to not encourage it. You can do this by getting him on a "SOLAR SCHEDULE" immediately!!! Please search the forum for "Solar Schedule", there are several detailed explanations of what this means and how to do it. Once you get him on a Solar Schedule he will be getting between 12-14 hours of uninterrupted sleep per day, he will have longer nights and shorter days, and his entire schedule every day will be dictated by the sun rising and the sun setting, just the way he would live in the wild. He should come out of breeding mode after 2-3 weeks of being on a Solar Schedule.
Also, do not pet your Nanday anywhere but on his head, face, and neck. That's it. No touching on his back, wings, under his wings, chest, belly, legs, or in the vent area. Even just petting him down his back once can get him very excited when he's in breeding mode, and he'll start to do the courting motions like head bobbing, beak tapping and rubbing, and eventually regurgitating.
It's also very important that when he starts any of these behaviors that you let him know that you're not into it. So when he starts head bobbing, tapping or rubbing his beak on you, or the head bobbing turns into him moving his neck/throat up and down and him looking like he's gagging himself, which is how they bring food particles up in order to feed their mates, you need to immediately put him down if he's anywhere on you. My Senegal parrot is just starting his puberty, and even though I have him on a strict Solar Schedule he still occasionally will start regurgitating for me, usually this happens first thing in the morning when I have him step-up onto my finger to say good morning. As soon as I see him start head bobbing I put him right back on his cage and walk away for a few minutes to let him calm down. DO NOT EVER YELL AT HIM OR PUNISH HIM FOR DOING THESE THINGS, IT'S NATURAL BEHAVIOR AND HE CAN'T HELP IT!! No need for punishment at all, you just have to immediately separate yourself from him as soon as you see him start doing anything at all like this, put him down and walk away for 5-10 minutes so he'll calm down. If you come back, have him step-up onto your finger or shoulder again, and again he starts rubbing his beak on you or head bobbing, then again just put him down, say nothing, and walk away for a little longer. The Solar Schedule really will stop this behavior before it escalates, right now your bird doesn't sound like he's very bad at all or being at all aggressive, but that's exactly what you don't want to have happen and it's what the Solar Schedule will prevent.
And one more thing, obviously if he starts humping you or any other PERSON you need to take him and put him down immediately onto/into his cage, on the floor, etc. and walk away. However, if you see him humping a perch, a toy, part of his cage, a stuffed animal, etc. I'm from the school of let him do it because it will help his frustration and keep him from humping people. I had a male English Budgie that lived to be 18 years old, and once I had him on a Solar Schedule I never had an issue with him being sexual with me at all, though we were very closely bonded. However, he had one of those plastic birds on a spring that you clip onto a perch and they bounce all around while the bird plays with them...He humped that plastic bird 10+ times a day at least! He would first sweet-talk the plastic bird, then tap and rub his beak all over it, talking non-stop to it the entire time while head bobbing like crazy, eyes pinning...He looked like a crazy, maniacal serial killer or something. Then he'd hump the daylights out of that plastic bird, lol. After that he'd hang out with me all day, watch TV, I'd give him head scratches and he'd talk to me, we'd eat dinner together, then I'd put him back in his cage for Night-Night, kiss him, tell him I loved him, covered his cage, and again he'd hump his plastic bird, then he'd get on his swing and go to sleep...
"Dance like nobody's watching..."
