Chances are he (she) is changing the tissues (skin) of the cere. (That's a bird's nose.)
If it really is on his facial feathers, could it be dried food that he isn't cleaning off properly?
Little female budgies ceres gets really strange looking as she is coming into breeding condition. Some look worse than others.
Unless the birds are kept outdoors there is very little chance they have mites. It's possible if the breeder kept them outdoors but it should have shown up before now.
Don't use one of those mite protector cans on the cage. That stuff is very toxic to breathe, for them and for us. They sell them everywhere but it's a money making scheme. Vets can cure mites with one dose of meds.
Annie