If a Clown is hosting in Zoanthids or Palythoa'a the palytoxin will cause black spots on clown fish. I've seen first hand where a trio of Ocellaris clown fish were covered in black spots while hosting in some palythoa's. Later on a Majestic Angel was added to the tank and he decimated every last paly and zoa in the tank. About 2 weeks later the 3 clown started losing all their black spots and they began to look like regular Ocellaris clowns again.