if they are less then 3/8" I'd get rid of them and not take the chance as they look like they could be those preditor pyramid snails you find on clams and other snails
cerniths usually have some brown in them
Pyramid Snails, or Pyrams are very small snails that belong to the Pyramidellidae family. They are generally white in color, have a slender shell that tapers to a point, and do not grow larger than a few millimeters. The Pyramid Snails use their long proboscis, which is a tube-like mouth, to puncture the mantle of a Tridacna Clam in order to feed upon the fluids and zooxanthellae cells contained within. A few snails will typically not pose a threat to a healthy Tridacna Clam. The snails reproduce every 90-120 days, and if left to multiply, can cause serious harm or even death to their host in a very short period of time