That is from an infection he had not to long ago. Anyway to remove it? He is in a quarantine tank and is doing fine as a matter of fact he ate a damn 9in barracuda right as i went to go check on him to see how they were doing, gulper is only 4in.
I have often used hydrogen peroxide or iodine solution on open wounds of fish if they show signs of getting infected. The problem is that (especially with scaless fish like cats) you can cause more skin damage fin damage than it is worth. If the wound is healing on its own I would not add to the stress on the fish by netting it, just add a bit of salt (1-2 tbsp/5g) or a slime coat enhancer like melafix.