Sounds like Rain Rot to me. My mare had it in the same spot, as well as under little scabs on various areas on her rump. It feels greasy when you start rubbing on the area. A week of scrubbing it off with a brush and some betadine, and you’ll be good to go! Be sure to keep it nice and dry. Rain Rot likes moist places. When you pick off the top layer though, and expose it to the air, it dies pretty quick. It is indeed a fungus.