Depending on the age of your clownfish, they can and will at some point undergo a color change as they get older, in which case they will appear to lose some of their black lines, will brown up a little along the dorsal area, and eventually develope new black lines. Does the fish look as though it's covered in a white dusty substance? If it does, this may be a sign of trouble (velvet/Amyloodinium) in which case I would recomend removing to a QT immediately for treatment if it's not already too late as this illness progresses rather quickly once external symptoms can be seen and hope that it hasn't contaminated any of your other fish. Does it look like it's covered with salt grains? If so, may be a crypt outbreak (Marine Ich/Cryptocaryon), again remove to QT and treat with formalin/malachite green as perscribed on bottle. If neither of these two apply, I would just keep an eye on him not too worrifully....is that even a word? And maintain your normal husbandry. It would help if you could include water parameters (pH, nitrates, nitrites, ammonia, water temp, etc.) as these could be leading to stress if they are too far out of range.