i agree, it looks like cradle cap in humans. brush/comb it thoroughly then wash with baby shampoo and rinse well and brush again. it usually takes a few days of this to clear it up.I would use a really gentle horse shampoo to try to get it all out. Just scrub it really well and be sure to put your finger nails into it. My older horse gets that sort of thing and it needs to just be brushed out and then bathed off. If it persist though maybe ask your vet.