There are all sorts of bivalves (clams, oysters, mussels, cockles, scallops, etc) that set up shop on hard corals. Actual ID would be difficult considering how well-encrusted yours is, but it's an almost certainty that it's safe.
That little critter is a living filter, and it'll process its entire body volume for nutrient constantly.
The coral will be fine, they grow commensally with bivalves all the time in the wild. Large bivalves are often 'anchor organisms', encouraging the growth of other organisms merely by their presence.