One more thing to check that requires grabbing it out of the tank and giving it a poke to make it retract: does the shell opening have teeth/ridges, and if so is it on both sides or only one?
If there are no teeth on only small teeth on one side of the opening, it's probably an Ovulid, which is a group I'm not too familiar with but that are in the iffy diet category (which includes gorgs and softies depending on the species). The elongated shape would suggest an Ovulid, but there is enough variation in the groups that the shell opening might be the better feature to look at. If there are small teeth/ridges on both sides of the opening then it's probably a Cypraeid. Small individuals are not always easy to ID, since (and I forgot this in my last post) some of them have striped juvenile forms that turn spotty as they grow. I also had one whose foot changed color as it grew, causing me much ID grief at the time. Fortunately most Cypraeids I am aware of are grazers/omnivores and not active predators of anything but the occasional small sponge or Tunicate. There are sporadic reports of people going "ahh my such and such cowrie ate my polyps!", but those cases are probably an act of desperation due to lack of sufficient other foods.