Ok, ya... funny I remember exactly which one he is... No offense but he did not look too good when I was there and I remember thinking about him. The problem is most likely your lights. Too put your mond at ease, I bet he was a gonner before he even made it into your tank. Lets see if I can explain what I mean.. An anemone is not suppost to be "white".. "white" is not a color that anemones are when they are healthy. When an anemone turns white is is because of improper lighting (among a few other things) and they have expelled all their zooxanthellae (symbiotic algae). When that happends they can not make food for themselves and they die. Even if you feed them on their own, they still need the sugars produced by the symbiotic algae for survival. Anemones have a very high matabolizm and can take weeks or months to die but rest assured under your current set up, your BTA will die due to the lack of light.
At this point, you got a few options to try to save him but I must tell you, there most likely is not much you can do if he is completly white. If he is still attached, you need to moce that rock to the highest point in your tank and get him some light. You can try to feed SMALL (<1/4") pieces of raw shrimp when the anemone is not all sucked in like he is now. Of he is not attached it might be even better. You can get a strawberry (raspberry) "pint" container from the publix up the road and put him in there. Fasten that to the top of your tank. This way if he does go south (die), you can pull him fast. Since you got him close to the surface, give him a good smell, if he smells bad (rotten) then you need to flush him ASAP.
Best of luck and if you have any questions, call me.