I have 2 of these little stars and never have bothered my zoas at all. They clean the rock around them though as you can see where they have been. I have always been told they are harmless in the tank.

I had a whole bunch of these in my last tank and I would always find them under my frags and around the base of corals , I manualy removed them over time ,
they were not just plain white in my case they were a moltey colour orange brown grey and sorts.

asterina come in many different varieties which explains the difference in colours, as for the number of legs some of mine have up to 7 but they reproduce by splitting themselves in half which is why you might see some lopsided ones with only 2 or 3 legs