[rynox77]

About half the size of a penny.

He moves around quite a bit on the glass.

 

[archer772]

Asterina star, some say bad but I feel they are good part of the clean up crew and thats about as big as they get

 

[phil_pl]

i have quite a few of these guys popping up in my tank, very fun to watch.

 

[ladyonyx]

I have a few in my reef as well and I don't think they're bad at all. They help clean things up

 

[BigandUgly]

Yup, looks like Asterina. They are a real common hitchhiker on live rock. I kind of like them. Some people say they can multiply quickly and take over a tank, but they are easy enough to remove.

 

[GetITCdot]

they are not "bad" persay but they will get out of hand very quickly, at one point i had about 6-8 hundred of them. They seem to die out and come back as they please.