Yes, remove from tank
For euthanasia, I’ll let you do your own research - as fish appears to be quite close to death anyway, it will likely seem to slip away with little response - clove oil is often suggested as a “home remedy” but after I carefully followed directions only to have some fish react extremely adversely .....
For the tank, as usual - change a lot of water, thoroughly vacuum substrate as much as possible, remove debris, clean filters ... optimum water quality always improves fish health
Consider any environmental stressors, and try to eliminate them
“Popeye” is more a symptom than a diagnosis - underlying issue may be bacterial, parasitic, previous physical trauma etc
After the major tank clean, I’d do large daily water changes (with continued attention to debris removal/substrate vacuuming (all surfaces really that may harbour pathogens)) for the next week
Then alternate day water changes and so on
If all fish appear fine after a couple weeks increased maintenance, assume any crisis has likely passed
Aquarium details?
Livestock?
It’s likely this fish has been showing some signs of illness (for a few days to weeks) but may have been playing least in sight etc, so try to spend 10-20min sitting very quietly near the tank and observe individual fish
If you feed a small amount, every healthy fish should be out as part of the ravenous hoard - depending on species, but definitely normal activity for most SA tetras, then fall back into normal behaviour patterns within a few minutes