They look like pictures I've seen of chitons- they are numerous and white, come out after the lights are off. Are they a problem, will they overtake the tank? How to get rid of some? I don't have access to a picture.
Are they roundish or oval? If round they are quite possibly Limpets, if oval they are Chitons (harmless herbivores). See if you can trade them in to your LFS. Usually without adequate foods (algae, diatoms) they starve to death.
I have had one of those in my tanks since the live sand was put it. It is really cool actually. He just pops up on the side of the glass under the sand bed and gets that layer of algae just below the sand line. Then hes gone for weeks and pops up again. Mine is going on 16 months and he is probably about an inch and a half.