It looks like it may be ich, if so you have two options:
1) Ich Management: continue to feed healthy foods and maintain high quality water conditions, eventually the clownfish will develop a tolerance to it. On the other side: if you introduce other fish, they may be more succeptible to this parasite and perish because of it.
2) Quarantine and treat: Move the clownfish and any other fish in the same tank to a separate quarantine tank and treat them using copper medications and water changes. You'll have to leave your display tank fallow (w/o fish) for 76 days while the parasite starves and dies off.
I currently have ich in my system and am sick of it. Any new additions have died within a few days or weeks. Im starting the quarantine process and going fallow. It's a long and tedious process but I feel it's worth it in the long run.
Additional advice: if you decide to go fallow, remember that any new additions to either QT or DT will cause your "clock" to reset back to day one. For instance: you've been treating and going fallow for 30 days so far and you decide to get a new Shrimp, that will cause your "clock" to turn back to day 1 and you'll have to continue another 76 days to ensure they didn't bring a parasite with them.
There are many useful articles in the Fish Disease and Treatment section of this forum. I suggest reading and studying those articles and threads to better make your decision.
Good luck on whichever decision you make! Happy Reefing!