They are harmless until something kills them. Then they ink and release some nasty stuff into your tank. While they do not eat coral the can attach to them and block light from the coral which is why I nuke them. The best fish to eat them is a black leapard wrass. They are hard to find and they don't eat much in captivity. You can also get a slug to do the job. i believe the velvet slug is the one. Some people like flat worms, but IMO i think that the negitives they cause far out weigh the postivies.