People asking about illnesses and diseases need to add a picture. These threads are worthless without one. How can anyone determine a disease from white film on him? Where exactly is it located. Is it white, clear, off-white? Where did it originate from on his body? Are there other markings? Is it swimming near top or bottom? Is it scratching? Does it have sores? Does it have stringy feces? Color loss? There are way too many questions, many of which can be figured out by picture/movie of the fish.
I can barely figure out what disease it is while looking and staring at a clown in person, I think it is a little presumptuous of the answers above to tell you what you have because nobody can know from that description. All we can do at this point is say that it may be internal or external parasites, but even this could be wrong as it could be a virus or infection. Too many choices, not enough description.
