Hate to say it, but I would go after it and isolate it in something I could control access, light, feeding, etc. I use large plastic pond planters for such duty. Maybe your set up will allow you to put one half submerged on top of a rock pile or to use some tie wraps and float it?
Anemones on the move tend to move, if this one has parked, I fear it may have given up. You might be able to susped a light or play with flow but you might just be convincing it, "I made a good decision to choose this spot."
If it was a 8-12" across specimen, I would be less concerned, they can afford much more time for trial and error.