There is more. Is it just the food? I first noticed it on the top of these two's heads. These are the youngest and still indoors...the feathering is very different then the other breeds we have. They are a bit neurotic too.
They're white and black beardless Silkies. The crusty stuff is just food, water, and other junk that gets on them, because they're chicks and chicks are messy. It doesn't help that Silkies have a crest, and it gets dirty really easily.
Honestly, Silkies' feathers are more prone to getting gunky like that because of their hair-like qualities. I don't think you have anything to worry about.