i would take that fish out and hold him in your net and pull whatever it is off , i doesn't belong there .i would use tweezers and just scratch the area clean with my nail . hopefully its just fungus or an infection that started because of the scale not falling off . if that's the case it should just clear up with salt and a little warmer water(make sure your fish dont kill each other),sometimes fungus is stuborn but not on a fishes side .i think that will clear up ...
