At webwetmedia there is a picture of a similar anemone that they claim is a
Phymanthus sp.. Here's the URL:
http://www.wetwebmedia.com/anemoneidfaqs.htm
It's the second one down I think.
Another possibility might be
Epicystis crucifer.
The best picture I've been able to find is in
http://www.reefs.org/phpBB2/viewtopic.php?t=65412&
(Looks like
Epicystis crucifer was once
Phymanthus crucifer)
HTH