They look like the sort of periwinkles that you'd find in local harbors and salt marshes here in Connecticut. They're algae-eaters, though I can't say whether they'll nibble on corals as well. They've been a problem, as they'll chew through the stems of Spartina grasses in the salt marshes.
I kept them years ago, and constantly had to pick them off the glass 0n the _outsides_ of my tank.
~Bruce