What are those tiny starfish---they are grey and literally a bit smaller than the stars your grade school teacher would put on your "conformist/assimilation chart"

These stars seem to start out unformed but eventually develop 6 spires (sort of like the "Star of David") They started coming out of my rockwork about a year after the tank was set up. I`d say there are almost 100 of them in there now. They usually hang on the algae on the glass.
Anyways........
What are they?