As I know - there is few of the small to middle sized hermits that really eat corals (if any) However they can irritate and move corals. Only the fact that you can see a hermit on a coral does not means that it is harmfull - it can be the oposite if it rinse away pest organisms from the coral.
hermits and other decapods like to play and pick at stuff so they can be pretty destructive even if they aren't eating corals. don't expect him to act reef safe, do expect a charming personality and a fun animal if there is no corals or algae to destroy
I mean he doesn’t look that big. Dad has a green legged hermit with white stripes in a shell the size of a baseball and he doesn’t eat anything just kinda knocks stuff over being a ding dong.