Looks like either a limpet or a sundial snail. Is he cone shaped, does he have a distinct point at the top of the shell? If yes he's the limpet otherwise he's the sundial.
Limpet are sometimes known to eat corals. Most times they just eat algae. I woudln't worry about him unless you see him eating your corals.
Sundials WILL eat zooanthids. I've seen it first hand. If you ever want zooanthids get rid of it. Otherwise i read they will die off without any zooanthids to eat.