Hi Julia, It's hard to tell what it is. Since you just got them I would start by guessing that it is an injury from inappropriate substrate. Is it getting bigger? Does it ooze liquid? I would start by treating it like an abrasion and cover it with neosporin. I know it is hard to keep this stuff on a frog but just check him frequently and reapply it as needed. If it doesn't start to get better soon than further investigation is needed by a vet.