Is this asterina ?

[joegoersch]

I've noticed these little starfish-like things on my glass. They are maybe 1 cm long and seem to be causing no harm. I don't know if the fish eat them or what, they seem to come and go. They don't seem to be messing with my corals at all..

 

[Kentech5]

Yep asterina. Some mess with coral, some are harmless. Can't really tell which til you catch them up to no good.

 

[Sapelo]

They also tend to reproduce rather rapidly. Find a way to get rid of them is at all possible.