Well the white scaly stuff could be rain rot, I'm dealing with that right now and it gets very gunky especially on old scars. Ring worm was already suggested.. but I have no experience about that to offer..
Discoloration could be some sort of skin fungus so wash it regardless.. and I'd at least call your vet and talk with them about the symptoms and if they have any advice.. beats a farm call.
We once had a bay Arabian mare with white spots on her back that we thought were saddle sores but then a chiropractor claimed it was dormant herpes but I'm not too sure that's what it is.
I don't know if it's just what some bays get after while since I am in no means an expert. But whatever it is, I hope it isn't serious.
I'm kind of bummed no one else is offering their advice.. but I hope I've at least planted a few seeds in your mind that'll get you on the right track.