Hard to offer valuable opinion/advise. What's been done so far? Details matter. Try this. 100% water changes every day, make water and age it 24 hours every day. 10 gallon TT.
MEDICATE THAT SUCKER. Addicted reefer knows what he's talking about. I personally would NEVER have another gig without medicating it. Period. My opinion. My experience. It's not going back to the ocean, face it. It lives, or dies with you, take care of it!
Try this. Change 100% every day, same time, make same way, every day. Expensive and time consuming, I know. Do this, and a couple years later, you will have a group you don't know what to do with, they don't die when acclimated. Don't listen to the guys that got it all together, the rest of us have sucky water, and it's hard to acclimate them to it. Don't use your DT water, make it fresh for your new nem, you're trying to acquire a new addition, and it's touchy! New, fresh saltwater every day is required to acclimate them to our crappy tank water. Once acclimated, they are very durable. Salt water, made same time every day, same way. Aerated. That guy looks, to me, like it should have no problem. I have a gig 3" big, captivity for over a year. Being ALIVE and healthy is more important than trying to grow it, like the "Jone's" anemone (""""""their anemone always looks so good, and grows so fast, why doesn't mine look like that!"""""") [I kid, I kid]. What is the point of feeding it to 24"? Want a healthy anemone, or a huge anemone? SMALL ANEMONE ARE EASIER TO SELL, TRUST ME! They don't have to be 24" to be healthy. My 3" gig is the same size today, only turned green from brown. Well over a year of captivity. I don't/rarely feed mine directly. DON'T FEED YOURS while acclimating, which can take months, or up to a year. yep, a year. Your problems won't be so big if it's not struggling to discharge all the uneaten food it can't process, while it's acclimating. Stop feeding it. Give it some fresh water to purge itself. Keep that flow up. When it's looking "AWESOME" FOR WEEKS IN A ROW, then, try feeding it some very small food, DUST, fed from a 1/4" straw. If it doesn't fit in a 1/4" straw it's too big. Growing it has risk, keeping it healthy has low risk.
They can die in a blink with low flow. You can keep them alive, literally, MONTHS, with high flow, even when you know it's dead, it can be preserved with HIGH FLOW, FOR MONTHS, even though it is dead. HIGH FLOW IS BEST. Did I mention high flow? High flow WILL NOT HURT THEM. Low flow, can send their internals into meltdown, and you may not see it right away. They come from low tide areas, that get POUNDED by flow. They can handle it. Sure, there's always the one exception that lives in low flow, but generally, they thrive with high flow. 11 of my 11 LOVE HIGH FLOW. I still have 11, hoping to rehome a couple in the next couple months.
My tanks are far from perfect, I struggle with my own issues, mainly time to maintain. But I can prevent meltdown by adding a powerhead until I can do tank maintenance. High flow, is really the easiest way to time buy with these guys. Gigs give you warning. I have so much tank rot and overgrowth in my tanks, it's really sad.
Dascamel, good luck. I think your gig still has a chance, if you work at it.