I've just noticed this today, but it may have lasted longer since I didn't observe him that closely for the past few days. The black part of his left eye seems to have expanded. My betta's eyes are usually like the second photo (right eye).
He's swimming steadily and isn't lethargic, but doesn't seem to notice things with his left eye. I just did a 100% water change. Is this something I should watch and wait on, or do I need to do something else?
Tank: 1 gallon
Water changes: One 50% and one 100% change/week with water conditioner
Temperature: 72-76 degrees (closest it will get with Hydor heater in winter)
Food: 3 pellets, 6 days/week; occasional bloodworm
Filter, water testing: N/A
Betta: no history of illness, ~1 year old