Do you notice any white spots on it? Real defined tiny white spots, on the fins and Gills, but could be every where. If so most likley ICH, you need to see the disese section of these messg. bds.
If not how is your water chemistry? I know in my FW tank my fish needed a conditioner in the water. It helps their slime coat keeps them healthy. How bout an anenomie or a coral in your tank? Have you seen him trying to host one if you have it? Im not sure how clown fish actually start to host but if he rubbed one and it stung him maybe hes a bit irritated. I know I had my Royal Gramma get a little banged up inside my LR when I was rearranging it and she came out with some scratches on her. She would from time to time rub on some thing. She doesn do it anymore but she did until the scratches went away.
Im not sure but those are just some ideas.