I would say that this is a Stomatella snail. They are actually quite reef safe, as they feed on algae. I find them in my coral tanks all the time from hitchhiking on coral colonies. If I had to guess, I would say that you probably have several of them in your tank as they breed quite a bit in our tanks. Take a look at your glass tonight a few hours after the light go out, I'm sure that you will see some on your glass. Like I said, nothing to really be concerned about, they're just like your other snails that you bought except smaller and faster.