About 10 yrs ago I put a rock with 5 mushroom corals on it in my 125 g reef tank. Slowly but surely they spread to other rocks, & attached to the glass sides. They overgrew & killed button polyps, annoyed a no. of soft leather corals causing they to detach from the rock they were on, and choked out smaller pieces of acropora.
I tried to reduce their numbers by cutting them out, using a kalkwasser paste, injections of Joe's juice-everything short of dynamite, but they still spread & flourished.
My strong recommendation to anyone with a reef tank is do not intoduce mushroom corals, unless that's the only coral you want in your aquarium.