Hi Zep,
It's not a large cerith, it's Bursa granularis. According to the (sometimes contradictory) info I've seen, species of Bursa eat worms, reportedly sipunculids or polychaetes/bristleworms, or maybe both, depending on the species. They spend most of their time hiding under coral slabs, but occasionally come out of hiding to cruise the aquarium.
Cheers,
Don