My guess is that the anemone needs to be acclimated to your lighting. Perhaps it came from a tank that was not as brightly lit as yours is. There are different ways to do this, including locating the anemone farther from the light, putting plastic fly screen between the light and the anemone, or reducing the photoperiod. I think putting a barrier, such as layers of fly screen and then slowly removing, layer by layer, over a few weeks is the best method.