sorry, need a better pic. from the pic it looks like an Astrea, or something like that, but from your description it sounds like a welk. the shell does not look like a welk shell, at least from the dark pic.
(wannreef...sorry for the slight hijack).........Geoff, you told me the other day that nassarius snails ARE welks. Does that also apply to the cool tonga nassarius snails that Marine Depot sells as "peaceful".
I bought a couple of them and they actually have a black and white striped snorkel as well as the ones i receved tfrom them also have a white and black and grey striped body...they are actually quite pretty.
definately a welk of some kind. hard to tell if it is a Nassarius or not though. since you do not see it whenever you feed the tank i would be inclined to believe it is not a Nassarius. i would remove it the next time you see it. Welks are meat eating snails. Nassarius tend to be scavangers the others tend to be predatory. may be taking out your other snails.