Here's another thought, now that I realize this is an established tank that you bought from someone...
How did you transport the tank from the previous owner's place to yours? Did you drain the tank and transport the livestock in buckets? Or did you just move the thing as-is? And did any of the substrate get stirred up in the process?
A couple things that I'm thinking about. Temperature is one thing - if the move was long, or if it took a while to get things set back up again, maybe the temperature dipped down for a while and that stressed the anemone? The other thing is that often when tanks are moved, gunk gets stirred up in the tank causing a mini-cycle. If you did have a mini-cycle, ammonia could've been present causing the anemone to stress. Any ammonia that was present would be gone by now, but have you done any water tests recently? What are your nitrate levels? If you don't know, I'd get a hold of a test kit and find out. A mini cycle might've bumped your nitrates up higher than the previous owner ever had, which could be effecting the anemone.