They're a fairly common hitch hiker - I've gotten some recently myself. Only 2 tiny polyps on mine, though. They're quite tough and can get invasive if too comfy. They would be good to practice fragging with if you need to remove some later. When gluing soft corals, you should try not to get glue on the soft tissues so getting a thin sliver of rock surface under the coral foot is the trick ;)
Enjoy :) the hitchhikers i usually get are Aiptasia *sigh*