At some point, I received this picture but I don't recall the source. These are pest Flatworms - aeolids - that tend to gather upon the surface of mushrooms, for example. While they may look like guests, they are definitely not helping the mushroom live. it is possible these feed upon the mucous of the coral, and are a general eye sore. They can be carefully siphoned off the object with airline tubing, or you could treat the tank with Salifert's Flatworm eXit. These are definitely unwanted.
Photo by unknown