Looks like a dry (injury) scab, so should heal. Mouth rot is usually inside the mouth. Is it inside mouth / gumline as well? That's where damage can be done. In a wet environment. Try to get him/her to gape/ open month to check inside.
It would look pussie, and most likely gray decoloration on inside of gumline and teeth. Q-tip can be used to see if slime/puss is on area just inside from outside mark.