Yes it is. I got the clown and one polyp when I bought the system a little over a year ago. About a month later the clown started to host the mushroom. The 'shroom didn't care for it at first and stayed pretty shrunk up. Must have gotten used to it though 'cause there are at least a half dozen shrooms on that rock now.
my LFS has a pair there now, 65 each, i was tempted after all the talk on here but only have one anemone and my fmale cinn clown still chases the male off a few times a day, so i think adding these would have been $$$ not well spent, but they are cool looking!!!