What Causes A Chronic Wart Type Growth On My Knee?

Hello, I ve had this wart type thing on my knee, it small but it has not gone away, it s been at least a year and a half, it s like a slight crust on the surface, it s not abnormally colored, but sometimes if I swim or take a long bath the crust layer will come off but it will stay and crust back over, and just recently it half way came off after a shower (the whole thing not just the crust) and now it catches on everything and it hurts and it bleeds when it catches something or when I try to trim it down. What could it possibly be?