Those, I believe, are nassarius snails. They are great snails to have in a reef tank because they help keep the sandbed stirred. They spend a great deal of time just under the surface of the sand looking for something to eat. Although they are not herbivorous (they won't touch algae with a 10" pole), they will clean up any uneaten food and other detritus. You may notice, at feeding time, that they come out and start looking around for food. They are definitely keepers and may even breed in your tank. If you see little clear bubbles about 1/8" across on the glass with tiny little eggs in them, then you're about to be the proud papa of baby snails.
Life is not measured by the number of breaths we take, but by the moments that take our breath away.