bubba, it's called urine scald or urine burn, and it's irritation and scabs caused by urine staying on the skin.
tselu, once you get it cleaned off really well, apply zinc oxide (a diaper rash cream works well) until it heals. Once it is healed, you can keep a layer of Vaseline on the fronts of the rear legs to prevent it from coming back, but if you do, be sure to clean it well and reapply often so it doesn't harbor bacteria in itself.
Bubba is somewhat right in that your horse may not stretch out properly to urinate. That could just be him, or he may have some back or hip and leg soreness preventing him from stretching. You may want to mention it to the vet and have them do a pain check the next time they are out.
"Keep a leg on each side and your mind in the middle"