Absent a test via veternarian it's hard to tell... They don't have to be in water constantly. As such you might apply an iodine solution to the affected areas (providone branded, at feed or some pet stores)and keep them out of water a few hours at a time and see how they respond. Fish remedies like malachite green or methylene blue in the water proper might help. Any change for the worse or spreading would demand vet intervention. They need clean (!) water; Moreso than alligators.