My horses have that too, as well as similar patches above their hock joints.
I always assumed it was from them getting on and off the ground. Like a callous. I've never worried about it at all, except for my foal who started getting sores near his hock joints from getting on and off the ground. So I put straw down in his favorite sleeping area and they seem to be healing up now.
I've never heard of "cradle cap."