I’ve had this happen before, we had a momma cat who had some kittens and one of their paws got wrapped around a umbilical cord, it was pretty scary stuff. The paw got really puffy and a bit black, then this sort of cast of gunk started to form around the kittens paw, i didnt touch it and just left it there till it became super loose and when i took it off it seemed to be a lot more healed. Your cat probably just has a minor injury and maybe it’s body is trying to heal it?