My chocolate chip ate my anenome after 5 months of having them both together, it took him about 2 1/2 days then it was a ball of snot when he moved off him. Yeah, NOT reef safe and i even tried to make sure he had some fresh krill/silversides twice a week.
Stars are so awesome, i plan to move mine to a seperate tank just to keep him once i start my reef. I like them so much.
About the live rock, he won't remove the corraline if thats what you wondered. knobby stars invert thier stomachs and dissolve thier prey, urchins have a scrapping utility and will remove calcium based algae (corraline) Maybe your lfs guy was confusing the two? I know how lfs people can know so little about things, granted they have a zillion other pets (at my lfs they have everything) Still no excuse though. Mine told me that the sea apple was part of the tube worm family. Haha.