The reason I say that it's unlikely that you have baby Nassarius is that AFAIK, all species in the genus have planktotrophic larvae. That means that the larvae have a prolonged planktonic stage (about a month or so) and that they need to feed on phytoplankton during that period. That poses several hurdles to their development in a reef tank. The larvae are pretty fragile, so being jostled around in a reef tank or pumped through filtration and circulation equipment is likely to do them in. They also make tasty snacks for things like corals and other inverts. The biggest problem though is the availability of photoplankton. There is very little chance of having enough phytoplankton available in a reef tank to keep the larvae fed, even if you dose daily.
Even in culture systems, it's a lot of work to raise most planktotrophic larvae in captivity.