I use Ivermectin 1% solution, and use it topically. Two or three drops for an average size chicken on the skin on the back of the neck. Repeat in two weeks. The good news is that even if this isn't depluming mites, it will kill any intestinal worms.
But there is a chance this is over secretion of sebum, as
@Wyorp Rock suggested, a fatty substance similar to dandruff in humans. Mixing a concoction of water and a dandruff shampoo and spraying it on the affected skin every few days would be something I would do in tandem with the Ivermectin since we're only guessing here, and if one treatment doesn't work, maybe the other will.