If that anemone is as white as the picture represents then it's expelled it's zooxanthellae and you have bigger issues than it eating.
Edit: The more I look at the tank it appears as if you either have a very new and young tank or you have had an imbalance and your tank is attempting to adjust to something severely wrong . Specifically I point that based on the diatoms all over the rock work.
Edit: Both of which should not have an anemone of all things in the tank.
Eating isn't your problem I assure you. Sure at this point it needs to eat as it can not get its food through the missing zooxanthellae. However, think more like the plant life surrounding Chernobyl Ukraine. Water is necessary for them to survive but is a bit of a side thought when in such an inhospitable wasteland.