no asterina stars should not be too dangerous but some species do eat zoas and soft corals. they multiply but not very fast. They come in white with orange dots, gray, brownish, bright orange, all red (rare), and maybe some other colors.
I'd say leave em be unless you have a single polyp frag of some valuable zoa. i remember once a few asterinas ate a good looking 3 polyp frag of one of my zoas. i was a little annoyed but w/e. I had thousands of other zoas.