Asterina starfish. Apparently they spread like wildfire and are difficult to get rid of if you aren't a fan. Harlequin shrimp will clear them quickly, but they only eat starfish so you'll need to consider rehoming, or finding a source of starfish to feed. Bumble Bee shrimp are rumored to eat them as well, but will also eat other "meaty" food, so that may be an option.