It is possible it has damaged the eye on something and it may improve without intervention. If the water quality in the pond is good I would be inclined to leave it alone while it is acting and eating normally. Often the stress of separating them is enough to trigger more problems :-?
If it worsens, or the fish's behaviour changes, I would remove it and consider an internal infection to be the cause. As it is one eye on one fish which is otherwise happy, I would be more inclined to think it an injury, rather than infection, at this point.