This is a common hitchhiker that you'll find in a variety of stony corals. This is a Barnacle, in the mollusc family, which will retract and extend the featherduster-like catcher's mitt used to capture particulates in the water. The coral in the background is a Galaxia. I've seen these in SPS coral, as well as Psammacora, Porites, and more. I've never observed any damage whatsoever by these creatures; rather they seem to be a symbiotic creature that lives in harmony with its host. I'd call it Reef-safe.