In addition to pyramidellids, some of the rissoids look very much like that. They are harmless. If you see your tiny snails hanging out on the glass and the sand, and not hanging off your clams or snails (mainly big, slow snails like Astraea and Turbo species) they are probably rissoids. Dr. Shimek wrote a good article about how to tell the difference between similar species in the two families, but you have to look at the earliest whorls (protoconch) under high magnification to tell the difference. In rissoids, the earliest whorls are in the same plane as the teleoconch, or the rest of the shell. In pyramidellids, the protoconch appears to be vertical, being ninety degrees off of the body whorls of the shell.
Identify This... Rissoid And Pyramidellid Snails - Reef Hobbyist Online Magazine
I couldn't get all the photos on the linked page to come up, but maybe it will be working soon. A rule of thumb is that the shell outline is often slightly convex in the small rissoids, while it's usually straight in the nuisance pyramidellids. In addition, if you hold the shell spire-up, the lips of most rissoids bulge outward in convex fashion, like this:
http://www.gastropods.com/9/Shell_12499.html
while those of the pyramidellids are more nearly straight.
Cheers,
Don
Any good snail ID links out there 
