It's hard to be sure from the pic, but...I think it probably IS a Stomatella. I blew up your pic and worked to clarify it, and the long antennae are plainly visible. Several snails have long antennae, but some of the most common ones found by aquarists are Stomatellas. It also looks like I can see the spire of the shell (see the circle right in the middle of the animal?) and the proportions are about right for that species. It does look like the animal may have a long skinny tail, though, and Stomatellas don't have long skinny tails. That's about all I can give you as far as an ID guess. Of course, you could catch it and take pics of it in a dish of water, and I could give you a definite ID on it.
Cheers,
Don
