I just found this on Lacey's back, right on her spine and I'm kinda worried about what it is. She's not bothered by a person touching it (I rode her before I noticed and she was totally fine) but if you touch it little clumps of hair with skin/root attached just kinda fall out. It's basically hairless skin where the hair has already come out.
Horses get really yucky dandruff in in the winter in areas like the back, where moisture can't get sluffed off. I think its a mix between shedding winter hair, and also a bit of a fungus maybe, but should go away as shedding continues. I've seen it many times. Once you shed the hair and the skin receives oxygen it'll go back to normal.