It COULD be rain rot. I've seen it come off with clumps like that.
Rain rot can appear as large crust-like scabs, or small 1/4 inch matted tufts of hair. There is usually dozens of tiny scabs that have embedded hair and can be easily scraped off. Underneath the scabs, the skin is usually (but not always) pink with puss when the scabs are first removed, then it becomes gray and dry as it heals. It is usually hard to differentiate rain rot from other similar skin conditions, so if you are unsure, call your veterinarian
I say it is Rain Rot, my new mare looked just like yours, she also had tons of tiny bumps under her fur, with 2-3 bald spots, and she shedded lots of dead hair where it was... I used MTG on her like everyone here told me to do, and she looks 150% BETTER so you might want to try MTG....
the big spot on her sholder is from rubbing its finally starting to grow back. When we did have a vet come up for a all over exam he never said anything about worms of any kind or anything out of the ordinary. Im going to try alcohol rub on the bald spots to see if it helps then maybe if I can order it again MTG.