Brown mushroom corals
These corals are easy to keep (with sufficient light), change shape radically during the day (in response to the light), and reproduce readily. When they get large enough, they elongate their stem, let go, drift around, and then settle down in another place. The original "root" then grows another head, and another coral starts growing. Eventually, they spread all over the tank.
A close-up of the texture of the surface of the mushroom heads.
Here they are with their stems elongated, looking very much like a newly grown mushroom in a forest. Notice that the stem is almost transluscent.
A mushroom about to let go and start wandering around the tank. Notice how long and thin the stem has become.
Here is a mushroom rock covered with mushrooms. The rock is in the center of the image and is somewhat diagonal, from lower left to upper right. This is just after the lights came on: the mushrooms are quite small. You can just barely see the white tentacles of a bubble coral in the upper right, and the bottom of a flower pot coral in the upper left.
And here is that same rock later in the day, after the mushrooms have expanded. The flower pot and bubble corals are now huge (at the top), and a toadstool mushroom coral has begun to expand at the bottom. The mushroom rock itself is invisible, covered with the expanded mushrooms.