No worries since they are harmless, not sure about the black slugs though :scratchch
Not sure about this but someone emailed me this link when I asked him about the asterina's...
check out this site GARF.org and especially this link
http://www.garf.org/news26p2.shtml#STAR
I have a lot of them as well and I feel like they are making my purple coralline fade...