Hi and welcome to the forum
Can we get a picture of the other side of the fish for comparison?
How long have you had the fish for?
How long has its eye been sticking out like that?
What other fish are in the tank?
Have you added anything to the tank in the 2 weeks before this started?
How often do you do water changes and how much do you change?
Do you gravel clean the sand when you do a water change?
Do you dechlorinate the new water before adding it to the aquarium?
What sort of filter is on the tank?
How often and how do you clean the filter?
How much salt did you add?
It looks like a physical injury where the eye has been caught on something and pulled out a bit. I might be wrong, but it depends on how long it has been like that. If it happened overnight then it's most likely an injury. If it happened over a period of weeks or months, it could be an infection under the eye.
If you added enough salt and it didn't make any difference, then you might need to try a broad spectrum medication that treats bacteria and fungus. If that doesn't help, then you might need antibiotics, but they should be used as a last resort because they can cause drug resistant bacteria that kill things.
Salt, you want to use 2 heaped tablespoons of rock salt, pool salt, or aquarium salt (sodium chloride) for every 20 litres (5 gallons) of water. Keep the salt in there for 2 weeks. If there's no improvement then use a broad spectrum medication.
To work out the volume of water in the aquarium:
measure length x width x height in cm.
divide by 1000.
= volume in litres.
There are 3.785 litres in a US Gallon
There are 4.5 litres in a UK gallon
When you measure the height, measure from the top of the substrate to the top of the water level.
If you have big rocks or driftwood in the tank, remove these before measuring the height of the water level so you get a more accurate water volume.
You can use a permanent marker to draw a line on the tank at the water level and put down how many litres are in the tank at that level.
Before you treat the tank, do the following:
Remove carbon from the filter before treating with chemicals or it will adsorb the medication and stop it working. You do not need to remove the carbon if you use salt.
Wipe the inside of the glass down with a clean fish sponge. This removes the biofilm on the glass and the biofilm will contain lots of harmful bacteria, fungus, protozoans and various other microscopic life forms.
Do a 75% water change and gravel clean the substrate. The water change and gravel cleaning will reduce the number of disease organisms in the water and provide a cleaner environment for the fish to recover in. It also removes a lot of the gunk and this means any medication can work on treating the fish instead of being wasted killing the pathogens in the gunk.
Make sure any new water is free of chlorine/ chloramine before it is added to the tank.
Clean the filter if it hasn't been done in the last 2 weeks. However, if the filter is less than 6 weeks old, do not clean it. Wash the filter materials/ media in a bucket of tank water and re-use the media. Tip the bucket of dirty water on the garden/ lawn. Cleaning the filter means less gunk and cleaner water with fewer pathogens so any medication (if needed) will work more effectively on the fish.
Increase surface turbulence/ aeration to maximise the dissolved oxygen in the water.