I saw this post and thought "hmmm, I had the same question a while back, lo & behold, I read part way through the original post thinking "this is awfully familiar sounding" just to realize that it *is* my post.
Anyway, noodus, our serpent stars are the same ones as the ones in your link. I have decided that they did not kill the fish, but enjoyed a meal as part of their clean up duties. They move around all the time, hanging out in different rocks. They have gotten quite a bit bigger than when I first posted, but we haven't lost a single fish since that post.