Hi there, are you certain that it's not any type of dirt, poo or food smudge ? If you haven't already, soak him for about 10 minutes [ lukewarm water ] and use a toothbrush lightly . If it doesn't come off it may be fungal, you can get betadine as well as Lotrimin anti fungal cream at a drug store. The Betadine can be watered down a bit + applied with a Q-tip 2-3 X a day, then apply the Lamisil at night. If you don't see any improvement after a week or 2 try to get in to a qualified reptile vet. , not just an " exotics " vet.