Maybe flukes from your picture. A freshwater dip could confirm and treat this infestation (immediate relief, but likely will not get them all), however I don't know if there are any special precautions with a Cowfish. Maybe another can provide some more information and insight.
Bacterial, fungal, and/or protozoan infections are strong possibilities as well.
Any other behavioral symptoms, like flashing (scratching, rubbing) or poor appetite, heavy breathing?