do these have irregular arms? if they do there seems to be two types of these stars. one that eats corals and one that eats biofilm/algae. if you find them on the glass at night than you are safe, if you find them at the base of your corals than not good. they are usually associated with SPS corals.
There are species of mini-britte stars that never get larger than a quarter. They are black, brown, white, brown and white striped, white and red, etc. many different color variants. They are beneficial to the tank, as they are detrivores and will keep the bottom relatively free of trash for you. The Asterina spp on the other hand, may (as Geoff says) be good or bad depending on which one you have...
I just bought something that sounds like what you are describing. A tiny little white serpent star. It's no bigger than a nickle with it's tentacles fully extended. The guy sold it to me for a buck. I put it in my tank, and doubt I'll ever see it again.
Here's a little sketch of what it looks like. Does anyone know if this is it's full size, or is it a baby serpent star? Sketch attached is actual size (depending on your screen size and resolution).
that is the one I found.. it's tentacles are very long, but yet it's body is very tiny.. it's white, and blended right on the piece of coral it was clinging to. It is an old hard piece of coral, not a live one.