Looks like what one of my girls had. Does it seem raised off the surface, and/or cloudy? In my case it was an injury, she was bashing her face into the divider and kept scraping her eyes up. I moved her to another section and the marks stopped showing up.
To treat just keep the water clean, double check to make sure the decor is safe- no hole is too small to get through, no rough ornaments, etc. Maybe add a little AQ salt if it doesn't go away in a day or two, or Epsom salt if the eye gets swollen.