That anemone is actually a white ball mushroom, as you said, Pseudocorynactis sp.
"the Corallimorphidae are mostly composed of temperate and deep-water species. The few remaining species that are tropical are sometimes availible to aquarists, but little is known about thier care requirements. Occasionally, they may be attached to live rock...Nilsen(1998)cover them in some depth and suggest feeding with fish flesh at night when the tentacles are extended."- E. Borneman's Aquarium Corals (2004)