If you see them one day with 7 legs and the other day with 3, its because they split constantly, they drop legs and another one is born, they are a pest. They usually just walk around eating algea off your glass or your sand, but, they also eat acro tissue, so i suggest if you ever see a few of these on any acros or corals, take them out. Everytime i clean my tank and i happen to see one, i take it out. Im always on the look out for any asterina starts on any of my corals. Its a possibility your linkia ate one. Asterinas are VERY slow, so your linkia must have just bumped into one and ate it..