Are they discoma's? If they are, they are really hardy. Pick a large fat one, and cut it off at the foot using a really sharp scissor. put in in a spot with good flow (but not too hard) and occasional use a baster to blow off any slime. It should heal fast and stick somewhere. if you do an iodine bath, it will be have a better chance.
If not, either chisel or scrap off. You could hang it upside down out of water and hope it falls off, or blast it with a pump and hope it dislogdges adn then put the loose mushroom in the other tank. Good luck