Toadstool Mushroom Coral
Here it is just after I put it in the tank. Note the short stalk and flat head, with virtually no polyps visible. The skirt of a button coral can be seen at the left.
A close-up of the surface of the top of the coral: note the dots, from which the polyps will emerge.
It is now hard to see the top of the mushroom at all, with so many polyps extended so much. The stalk is hidden by the flower pot coral, at the lower right.