It is rain rot, mud rot, scratches, whatever you want to call it. Try to keep him out of the mud or wet areas, give him plenty of bedding, and when you bathe him, make sure to dry his legs thoroughly. This is common in show horses because they are bathed all the time and don't think to dry the legs off. All of the corrective suggestions listed above should work well.
Those are different things.
Rain Rot is caused by a bacteria on the skin. It produces little round scabs.
Mud Rot, Mud Fever, or Scratches is a skin infection, usually located on the back of the pasterns under the fetlock. It's produces an area of scabs and irritated or weeping skin and is warm to the touch.
The white gunk on the lower legs (between the hock and fetlock) does NOT have irritated or scabby skin, unless the owner/handler scrubs it too hard. The ONLY symptom is the production of white flaky dandruff-life "stuff" from the skin. The hair doesn't come off in chunks like rain rot either. The hair will shed like normal, or slightly more pronounced shedding, but the horse won't go bald. Private Discussion
From the vet: "Well I think we have two related problems here. There is a definite condition of greasiness without hair loss that occurs on the front of the rear cannons that does not seem to be an infectious disease, more of an overproduction of sebum and the hair is not lost and the skin not irritated underneath. I am as much at a loss to explain why some horses do this though males do seem effected more. Shampooing is the only reasonable course and antiseborrheic shampoos seem logical." (dandruff shampoos)
Oh, and curry combs are fine on the lower legs. Just go with the direction of the bone (up and down) and don't curry to hard across tendons. My horses LIKE being scrubbed with their curry on the front and back of their legs.