Looks like lymphocystis. It is rare in clowns but it does happen. A viral disease usually brought on by stress. There's no cure, but it is usually not lethal. My Juvi emperor had it when I got him from my LFS and showed up during quarantine. I took like 3 weeks for it to go away, but my emp was eating and behaving normally other than the white lump on his prepercular spine and his tailfin. He is has now been with me for 3 years and is a full adult. In cases where the fish is no behaving right, like not eating, hiding, and overall scared to death, you may need to put him in quarantine by himself with hiding places. Some people just "scrape off" the white lump with tweezers but if the clown is really stressed, you might just kill him. I've heard paraguard is used sometimes, but since it's viral, I doubt it has any effect.