If you do a google image search on 'Black headed ringneck', you'll see photos of birds who look similar to your Alex. Apparently, the mutation has been noted among ringnecks, but is not common or readily available. Your bird might be a ringneck/Alex hybrid or an Alex who is showing an amazing new mutation. Either way, a checkup with an avian vet is a good idea. You might also get in touch with an Alexandrine or Ringneck breeder to see what they know about it. Please let us know if you discover anything. I think your bird is gorgeous and if his black feathers turn into a solid black mask, he'll be stunning!
Duh! I just computed that Iago is eleven years old. Now that
is odd! It might be something in his diet or something in his environment that's doing it. Ask your vet!