This is a Whelk, a snail that consumes dead livestock on the reef. It will also attempt to attack and kill snails. These are carnivores. I had at least six of these in my 55g reef for the past year, and never had a problem. After acclimating a baby Golden Maxima clam, the next day I found all the whelks finishing off my beautiful clam. They definitely attacked it - as clams are part of their diet - I only learned later. (sigh) NOT Reef-safe at all. It looks like a Nassarius snail!
http://www.melevsreef.com/id/whats_a_whelk.html