I keep seeing this on other frogs through out the web, too. Most of them were from injuries and some turned into infections because of the injury. These guys have very delicate skin and are easily injured. I am not sure of how to treat something like this though. I would contact your vet if the spot does not heal up quickly. Good luck to you and I hope your little guy is ok? If I find anything out about the treatment or anything at all, I will post it here for you with a link.