Asterina species star.
They'll reproduce by fission, so you often see them lopsided because they're regrowing their arms on the split side.
I've never been bothered by the ones in my tanks but some people claim they are not 100% harmless, like they might eat SPS polyps. I've never witnessed this myself but there are probably differences in preferences between individuals and different species and whatnot. Although it bears mention that I'm completely unable to grow SPS.
But I'm reasonably certain it's not these stars that are to blame.