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. Some folks know this as "urine scald" which is certainly possible for geldings and stallions but very unlikely. 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. At the end of the day, seborrhea is basically mild, shouldn't spead to other parts of your horse, and is very cosmetic in nature. Because the cannon keratosis is just over bone, I prefer to groom it with a soft curry mitt and then a soft brush. You can use the same soft curry mitt in the wash rack with a mild shampoo to remove the gooey stuff if it's really bothering you.
Keep in mind that loads of skin stuff can look alike. Rain rot, sweet itch, and other skin infections can be distinguished by your Veterinarian and the proper medication can be prescribed if needed. Cannon keratosis can be managed through good grooming, but still warrants a look from a Veterinarian. If it is particularly stubborn, spreads, causes secondary infections, or you suspect that it's not cannon keratosis at all, call your Veterinarian and see what's up!
How do you deal with cannon keratosis?