Are the frogs housed together? If so, It's possible maybe the other one nipped at it. My frogs get something that looks like this when one gets a little too excited and tries to bite the other. It's just a surface abrasion and goes away in a few hours. I'm not an expert but as long as he is fine otherwise I would just keep an eye on it. If it gets worse, maybe some Neosporin without pain killer would help.