In my experience there are the 6 legged ones and the 5 legged ones. The 5 legged ones are the ones I have seen on the bases of coral. Some say they are only cleaning up dead tissue but I still don't trust them. Now the 6 legged ones I have only seen on the rock, sand, and glass. As a matter of fact, you hardly ever see one with all 6 legs the same size. They split in half and the "splits" have 3 big legs and grow 3 more small legs.
I've got a couple of Harleys in one tank that keep them under control. The other tanks have big wrasses in them and I think Harleys would just be a snack for them.
I've had Harleys clean up a tank so you couldn't find an asterina. Remove the Harley and a couple of months later you start to see one or two. I think they're like roaches; you never completely get rid of them; only keep them under control.