I've been having some issues with asterinas on my high end small zoas. I've been manually removing ALL asterinas small big white dark. I've easily pulled 100+ in two days about half hour each day.
My harlequin loves them too and I love the harlequin so a win win. I actually found that my zoas were not doing well when I had lots of them. Then again, I had a foxface that may have been snacking on them zoas too. So, not sure. Keep an eye on it and if zoas or anything becomes an issue, get them out of there. I have heard they can be pretty nasty (certain ones).
There are hundreds of different types of Asterina, identification to each is difficult at best. We can make generalizations like blueish colored tend to eat coralline, pink and red sps eating. Number of legs of no importance since one method of reproduction is lateral fission. Add to that the dynamic of opportunistic feeding (they are eating what is available but not the typical items they eat in the wild).
Personally I really like them but I have had them eat zoanthids outright. Now if they started munching on my sps I might feel completely different.
If you have lots of asterinas in your tank and you get a Harlequin shrimp, do you still have to buy star fish to feed your Harley, or will the asterinas repopulate fast enough for the shrimp?
I have hundreds of them in my tank. I don't much care for the coralline, so no loss there. They also don't bother living tissue on corals. the colored variant do, not the grey ones.
I've had a very thick population for the entire year my tank was set up at my Dad's house. They were white with a tiny bit of brown on top. Never bothered a thing.
I say 'were' because I ended up killing the majority of them during the tank move.
i had them for years actually thought they were kinda cool, until one day i noticed an explosion of them when there were alot they ate my expensive zoas and palys left the cheap ones alone.. after that i made it a point to get rid of them all lol..
I have never had issues with the white ones the seem to try and keep algae under control. I have a lot of Zoe's and plays in my tank never seen them attacking them.