What is that leg funk that can "grow" on the hind legs? Usually seen on the front of the cannon bone of the hind leg, it's also known as stud crud. It's not itchy for your horse, and it's very common.
Stud crud, or cannon keratosis, is a skin condition caused by the horse's own glands in the skin. It manifests itself in a waxy paste that sort of glues the hair to it. It is not fungus, bacteria, scratches, or caused from urine. It's a case of seborrhea (think flaky and scaly), and it can be primary or secondary. Primary (which is stud crud) is what you find on the hind legs, and isn't associated with another infection. Secondary seborrhea is associated with a skin infection, a nutrient deficiency, liver disease, and a few other things. It can also be associated with oily or dry skin.
You will need a Veterinarian's help to determine what your horse's leg funk is (primary or secondary) and what you can do to treat it. Likely, your Veterinarian will be able to give you a special shampoo, probably one that contains benzoyl peroxide.
You can also do some things to help, like only using clean and dry boots, keeping your horse's legs clean, and washing and drying thoroughly with your prescribed shampoo.