They are probably little serpent stars. A while back I got some green star polyps that wouldn't open, I saw little arms coming out so I did a quick dip. A little serpent star came immediately. I don't like serpent stars, they will eventually cause problems with fish. When they get big, most experienced reef keepers consider them to non reef safe because of their habits of catching fish. I was always told they were reef safe, but after a couple of my favorite (and most expensive) fish disappeared, I realized the culprit. He was a master fish catcher.