Anenome have slow metabolism and by the time we realize that they are in trouble, they are in a downward spiral.
How bad is he, white striations near the oral disk or totally, whitened tentacles?
You did the right thing getting it into MH, most people are stubborn with the PCs and never realize they don't have enough light. Kudos!! Guilty of it myself by the way.
It is a slow road to recovery for this guy. Assuming the water, and current is agreeable, he should recover with these Halides. What is needed is to feed more than regularly, (once a week to two weeks is regular), up to every couple of days bordering on the force feeding. You don't want him to get so bad as to stop feeding.....The eat slow also, kinda like a spider sucking out the good and spitting out the rest. Feeding will increase the process.
Within time, with regular feeding and the new light should be fine.
Happy anenome do not move, are not white, and have tight closed mouths, not gapping.