It's not life threatening. If it's only on one side, it's usually due to mechanical injury. If both eyes are affected, it may be a bacterial infection. If it's only one eye, you can ease the swelling with epsom salt. But that's really all you can do. It will need time and good water quality to heal on its own. My flame had this a while ago, took about 3 weeks for it to go back to normal. I don't remember exactly but I think I used one teaspoon of salt per 5 gallons of water. Good luck.