Ok. You’ve established several things. You are an educated reefer. I too, have moved tanks, and have put anemones in a moved tank, and a new tank, it CAN be done, but only by those that are dedicated. You have proven you are dedicated.
So, all crap and cordials aside, that anemone can still be saved. To my eyes, it looks like a gig. No one has mentioned that. They can be tough to acclimate, they’re different than haddoni. Are you certain it’s haddoni??? From the pics you’ve posted, no gig would do well in those locations, regardless of water quality. Does it have dots under its skirt? It’s pretty bleached so dots might be hard to detect. I’ve had both haddoni and gigs. I only have gigs now. I’m familiar with both.
If I was you, pull that guy. Clowns and crabs will make it worse, until they’re healthy. Put it in a tank by itself, do 100% changes everyday. New water everyday. It’s best to have water made and let age 24 hours before using, there’s chemical balancing that happens that no one talks about after a day, for new saltwater. Use an airstone. Oxygen levels affect gigs. RODI is best, I use only RO. I’m guessing, that guy to me, doesn’t look like needs treatment. Needs a flush. Just my best guess. Change water everyday for a few days and observe progression. Get meds incase, but I wouldn’t use them since you’ve had it so long, I suspect it’s an environmental issue it’s not happy with. Not chemical, since your sps are doing well. That said, there are bacteria that go through cycles that happen, and they affect anemones differently than corals.