I got this reply from Dr Frye:
"That appears to be a mild or early case of bacterial or fungal dermatitis. I would recommend treating the frog with Baytril drops and SIlversulfadiazene."

It feels good to at least know what it is! I will do as he recommended and hope that my frog gets well.
Do any of you know if this can go away by itself? I wouldn't take the risk but it would be good to know anyway.