He will eat grit when he needs it. They only need a very small amount and he may be getting it from the dirt/ground. The grit is retained in the gizzard until it is broken down itself to small particles and is then pooped out. They instinctively know when they need more. Cutting back on treats...
I believe certain wormers can cause damage to new feathers if used during moult. If you didn't worm them recently then I would agree that it is probably a nutrient deficiency. I don't think depluming mites would make such a regular pattern as to create translucent bars like that.