purple polyp? coral id needed

got this for free from "a pet store" in sumter sc last week, im guessing its a type of polyp but not sure what its called. i was told it will take over the tank so it was free for a hand size piece so of coarse i took it!

when its closed at night it looks like corraline algae on a rock, nothing sticking up just looks like corraline algae on a rock lol.

someone just told me it may be a star polyp, when its closed its like bumpy corraline algae on a rock.

lol yeah i know, i forgot about the closed look.
its only about 1/4" tall now
will it get any taller or will it stay short?