Tizzo is correct, they are mini brittlestars and they will not get any larger. They are not likely to eat the polyps, but if you have a lot of them there is a possibility that you are overfeeding your tank causing other issues.
What are your specific water parameters? What lighting do you have? I think you need to investigate other reasons that your polyps are dying. IMO, no, it is definitely not the brittlestars.