Xenia: well that did it on its own in my tank. The only way I can remove it from an area is to take a razor and cut it off the rock. If I simply use scissors it will just regrow back. I do like it, but they spread everywhere, even stick to the sand & glass for me...
Mushrooms: Well I haven't had a whole lot of luck spreading these. The one I had made a new one itself, but that was only one in about a year, not really weed speed. The one trick I did with another mushroom was to tear it from where it is stuck. It will stick to a new place and one will grow from where it was. I was one told if I take a mushroom, slice it up, and drop the pieces back in my tank, it'll spread. I never had the heart to try it.
Kenya Tree: Mine has dropped a piece and it grew where it fell on the rock. Since then I took sharp scissors, cut off a branch, and where it laid, it grew. I've done this a few times, and no real harm to the main coral. It shrunk down for a few hours, but bounced back.