Nice try. Definitely isn't a mushroom. Here are some more pictures on web pages:
http://www.tomuraya.co.jp/wakamiya-10.htm
http://www.kt.rim.or.jp/~sokohaka/chin-man/kanamara_m.html

There's even a comment along the lines of "The foreign tourist is really getting into it."

Extra points if you can spot the demon version of the female equivalent (hint: it is a painting in the second link).

I have always meant to go to this festival, and this year I just might have to!