It's stressed and that's why it's losing the tips of it's arms. Could be starvation, could be some recent acute change in water conditions. Not predation. I've actually had sea stars ordered online arrive this way already losing tissue in the bag, even with overnight shipping, and fromias like this guy seem to be especially sensitive. That said, sometimes they will lose part of an arm, you think they're gonna die, but then they seem to recover and regrow the arm; I've got one right now that lost about half of one arm, but now the tip of that arm is healed up although still shorter than the others.