Your using RO/DI water yeah ?
if not thats also where po4/algae could be coming from.
id say if you get all these things right in the next 2 months or so your anemone "may" even come back in the months too follow after that with proper feeding .. but it will be a very slow recovery & its hard to tell as they have a very slow metabolism and it personally looks too far gone .. maybe remove it also as an anemone dying in a tank has the potential to crash it from what ive seen in other systems in the past where a bta goes through a powerhead etc while people are away all to come back to everything dead.
once you get these things right id go ahead & add another bta and feed it once a week and should flourish.
its not so much that theres something wrong with the anemone its the consistency of water quality around it.
They need proper consistency.
if you have clowns try elegance/goniopora/torch etc mine host my elegance atm till the tank is consistently stable enough & ready to add a h.magnifica.