Dang your camera is nice, nice start for your setup as well. Clean lines, nice substrate and rock. Although I'm no ID master, I'd peg that little star at 99% assuredness that it's asterina and not all that harmful, at least not a polyp hunter in my experience. Heck, I'd cut him in half and make two! These guys are your common refugium stars by the dozens in larger setups... I used to pay some guys a few bucks to collect them out of their tanks and given them to me as centerpieces for my pico reefs. I like the warning to ID any unapproved hitchhikers though, burntom, as you never know who'll go after your prized zoanthids or stony corals.
B