2 days ago my Anemone got stuck in the filter and died. The water was cloudy a bit and i changed 30% very quick. Yesterday i noticed my only damsel not eating and not swimming. he is staying low on the bottom of the tank and stand there.
When anemones die they release a lot of toxins that can nuke the tank, which is probably what is wrong with your damsel right now. Im really not sure there is anything you can do for him except more water changes and hope for the best.
I didn't post before because i didn't have time... But the damsel died at the same day.
For anyone that will have this problem in the future and will get to this thread for help, please be careful. If your anemone die or start making your water cloudy white, take care of it right away. I didn't do any water change when it happened and i got my fish lost.
Take the anemone out of your tank easily, too many movements will cause more toxins to spread. Change water everyday for about a week. As mentioned here, the first water change should be more than usual, but not too much (about 40%). then, 20% everyday. Check the water for amonia...