Take it out of the water, let it dry off and apply either some Betadine or Wardley Fungus Ade. Leave it out of the water for a few hours then put him back. Repeat for a few days and that should clear it up.
Others may suggest an aquarium salt bath at a slightly warmer temp than usual, this will also fix the problem, but make sure you fix the cause of the problem, ie you might need to do a water change.
Allan
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