Certified GKH Gardening Expert
It appears to be a fugal issue. Spray the tops and soil with a few drops of one or more (if not all) of these ingredients mixed into warm water and a spoonful of coconut oil: Rosemary oil, garlic oil, clove oil.
You can even mix half peroxide, and half water next time you water. This will help kill off anything in the soil, and help oxygenate the roots. Most people don't realize that ,although the plant breathes and uses co2, it also needs oxygen at the root zone.
This will bring back your mint. Also, it would like to be in the brightest window possible, or under horticultural lighting. Watch how much water you use, and keep the soil moist. Not wet or dry, but in-between.