I think I have a bristle star.....

[9supratt4]

I got a piece of coral on a piece of rock and I've since noticed arms coming out from the rock and grabbing food. I think it's a bristle star. It has stripes on it's arms (black and tan) and what look like hairs on the arms. I've so far seen about 4-5 arms but no body, I haven't seen it out of the rock.
Are these things good or bad??
Here's some pics:

 

[mr_x]

they are good. he'll not grow to more than the size he is now though, and chances are, he'll stay half in and out of that rock for his whole life, grabbing at any food that happens by.