i'm interested to know as well. looks really cool.
wow that's beautiful ,looks like a macro I tried to ID it but couldn't find a positive match
I found something close its in the caulerpa family called saw tooth but I'm not positive that it is correct , this is what I found , don't know if it's the lighting on the plant in your picture that gives the 2 shades of green or if it really like that
Marine Plants in the Aquarium
Stichodactyla gigantea. That is an anemone that is parked a top of a sps colony.
Copied the wrong name! Sorry about that. It's a feather star. Cenometra bella.
Been awhile since I've seen a picture of one. Here about a year ago saw some in tanks and people were going wild for them...until they learned their abysmal life span in the home aquarium.
Edit: Found the exact guy you saw out of a UK site. It'll be pictured down at the bottom of the page.
http://www.amoenitas.co.uk/marine5/marine.htm
Double Edit: Here it is called a Bennett's Feather Star
http://reefguide.org/oxycomanthusbennetti.html