Please give us some history about the fish and the tank(s) so there is some tangible information that may help shed some light on this.
How long have you had the fish?
Where they QT'd? If so, how long? Describe the QT set up/water parameters.
Where any treatments performed?
Describe the display system along with water parameters, chemistry and maintenance.
What fish are they currently housed with, what types of mobile and/or sessile inverts?
Did this suddenly come on or has it been there awhile?
What exactly do you feed, how often and are there any food supplements used?
Any information along those lines will help.
The most common skin anomalies on this type/species of clown will be a form of melanism. That can either be caused by a chemical reaction to a host (usually a strong chemically defended soft coral), improper diet or overall tank/living conditions. Lets not jump to any conclusions that quickly though.