They are Sand Sifting Stars and they do cause a little bit of trouble some times but only with the rocks that lay on the sand. I have more trouble with my Turbo Snails though.
I would not recommend the Sand Sifters though. Now that I have them I find out that they eat all the good critters that are in the sand and when there are no more left the Stars starve, or at least I have been told happens. I am hoping that they will eat any left overs that the fish do not eat.