Asterina. They can be all sorts of fun shapes and sizes. The arguments for keeping and removing them are well documented on the web. I've decided it's more of a personal choice. Most likely they don't harm anything, but people have had issues with a few types.
Most people remove them. I've been taking out any I see.
Possibly Asterina? Hard to tell the size from the foto. I've not seen one much bigger than a 1/8". I've heard mixed stories about them, and do know they reproduce by division. Some say they all eat corals, some say only the colored ones do and that the white or grey ones are reef safe. I had two in my 14g that came in on a trochus shell and got rid of them just to be safe. Strictly your call. They do add biodiversity. They also reproduce fairly quickly if the water params are good.
I'm going through an explosion of them in my biocube. I remove a few everyday. I might get a harlequin shrimp but it would be tough to catch it later to remove.