The two common causes for popeye in marine fishes is supersaturation (unlikely in most tanks, but maybe Fluval Edges, with their different design, might be prone to that).
So - mechanical damage, I wonder if the mantis is involved? Depending on their relative sizes, and if the mantis is a spearer or a smasher, it could be. I normally keep mantis by themselves.
Maracyn two is a could broad spectrum antibiotic, but if the eye is not cloudy or red, there is not likely a bacterial infection involved here.
Jay