Mmm, your toadstool is more valuable. Leave it on the plug. A sharp pen-knife may serve to abrade the surface of the plug and scrape the xenia off. Xenia can become a tank-wide pest unless you are willing to monitor where they go and take ruthless measures to confine it. Scrape it off, and then set it on the bottom sand and lay a not-too-heavy bit of coral rubble or rock on it. By morning, turn the rock over, and it MAY have adhered to the rock for a happy second chance. If it demises, well, it shouldn't be too expensive---but I don't think it will. Sounds callous, but xenia can be quite, quite tough, and is far from endangered. It's pretty stuff, but in a well-balanced tank it can get really aggressive. OH! and put a carbon sock into your water flow before annoying either your xenia or the toadstool: they spit nastiness, which dissipates fast in the ocean---in our circulating tanks, not at all. That will absorb it.
