There are a few Miters (family Mitridae) that may feed on small mollusks; however, the vast majority feed on worms, especially sipunculids (peanut worms). The species you mention, Mitra mitra, does go for worms. I've witnessed this first hand while diving. On one particular occasion, I pulled one up from the sand with its proboscis still extend (longer than the shell). It was in the process of consuming a worm.
All miters are carnivorous and will not eat algae. Honestly, I don't know why these are turning up in the aquarium trade as I doubt most reefers can provide for their diet.
Cheers,
Bob
