white substance in mouth

Was playing with Sierra today and notice a white(almost clear) substance on her beak and in her mouth.

If it's not something she just ate, and she wasn't regurging to you. Then yes it can be a sign of yeast infection or bacterial infection. In which case she would need to see an avian vet, even if you haven't noticed other signs of illnesses. As I'm sure you are aware birds can/do hide being sick, this is because sick birds in a flock will be kicked out of the flock or killed. So if you are seeing something abnormal it's best to go to the vet

My bet is on yeast infection. Observe her droppings closely and monitor her weight. You can notice a consistency change in droppings (i.e. bubbles, or "frothy") and generally parrots will loose weight as well. But, they can also not show many symptoms until it's too late. Take her to the vet, but short-term you can try adding Apple Cider vinegar to her water to at least help her out. It should be a very small amount, like half a teaspoon per cup I believe.

Was she trying to feed you? Have you seen this before? Was it anything new in her chop? Is she taking any supplements?
If you answered no to each of the above questions, your bird needs to visit the AV. Try to get a sample of it AND take a pic. Very possibly the AV can resolve this by text or phone.
It's possible your bird may be reacting to medications or supplements.
Something to be aware of is possibly a Vitamin A deficiency. Another possibility is either a nasal discharge or bacterial infection. Depending on your location, allergies may be affecting her.
As you see, there are several possible reasons for this. Without actually seeing or testing, we can only make experiential comments and guesses.

Hey, Stephen! Are you able to post a picture? How much substance are we talking about?

I have been watching her closely. She hasnt done it again since that day. Weight is normal and so is the droppings. She does have her new feathers come in on the back of her head and neck. No sure if her being in this stage plays a role.

I'm not sure what you're describing here, but ANY type of discharge from a bird's nostrils, whether it's clear or not, is a sign of either a sinus infection/issue or an upper respiratory infection/issue. The reason that this is a big deal is because, and I'm sure you're aware, birds hide any and all outward signs/symptoms of illness and pain for as long as they can, it's a natural, innate survival instinct that protects them and their flockmates from predators who target weak, sick birds. So when it comes to both Sinus and Upper Respiratory Infections, a completely clear discharge from their nostrils is a very early sign; The same thing goes for a white discharge/film from or in their mouths or around their beaks/nostrils as an extremely early and first sign of a fungal/yeast infection. So you may have actually gotten lucky and been able to see this very early sign, as opposed to how it usually goes, where your bird is sick for weeks or months and you have no idea until they are extremely sick and start showing other, more serious signs and symptoms. So it would be a very good idea for you to take him to his Certified Avian Vet or Avian Specialist for a simple culture so that you catch it very early and get him on the correct medication for whatever microbe is causing the infection, rather than waiting until he's really sick and requires much more invasive and much, much more expensive testing and treatment further down the road.