I don't know WHAT that is, possibly some kind of fungus? If you don't hear from somebody today that has a clue, I'd take him/her to the vet to get something to put on it. If you find out, please post what it is. I'm really interested in finding out myself.
It looks like a scab, from some sort of injury to me, but I would also bring him in to the vet to get a scrape done to make sure it is not fungus, and if it is not, maybe some cream or something to help it heal.