there are several kinds that are hitchikers most are alge grazers or detrivores. but there are a few that may go after corals. most likely it would be ok but keep and eye out just in case. i have some that came in a detrivore kit that are white with several legs. they were just labeled as micro stars
Here is a pic of a very small star on some carbon. Did it look like this. if so also harmless.
The Picture that karlas posted is a type of star that some consider bad for the coral reef tank. I have never seen them on a coral but I remove them if I see them just the same.
This is the small serpent star I think that should be safe for the reef tank. these little guys multiply like rabbits I guess because I have them all over at night and under every rock.
It has the shape of a britte star. It's center is round with the legs attached to the sides not like the star that you mentioned Jawbone. To small to take a picture of. Maybe it is some kind of hydroid? Just a guess.