Could you please offer us a few more details?
Tank size?
Tankmates?
Decorations used? (Perhaps a full tank shot will do it.)
Water parameters (ammonia, nitrite, nitrate, pH)?
Test kit used?
Foods provided? How often do you feed?
Tank maintenance regimen?
Filtration?
Temperature?
Any other unusual symptoms?
Please include a photo of the affected area. Until we know the exact issue, please be advised not to treat your fish with anything other than doing water changes as necessary.