i think they are little star fish but all you can see is one leg sticking out of the live rock...they are all over the rock.....is the a fish that eats it?
They are Micro starfish and they are perfectly fine. They harm nothing and you really don't want to get rid of them as they are excellent scavengers and will eat the little tiny bits of food that get stuck in your rockwork.
You will often see them exhibiting the behavior of waving there legs immediately after you feed the tank, or when they are spawning. They will not 'overgrow' the tank as they will only populate up to the level that there is food available for them and they never get any bigger than about an inch in size