The fact that he is that old and has been in a 3gal is the saddest thing ive read in a long time. But it is a decent sign that you at least stay on top of water changes. Please upgrade that tank as soon as possible and research how to properly care for your fish. That fish needs a min of 20gal and ideally at least a 40gal. Its as sad as placing your dog in a kennel their whole life.
The stuff you see on your plants is one of the reasons not to use fake plants, you never know what they have on them. I agree with liberty, it looks like a fungus of some sort. This could be growing on the plastic plants OR leftover food.
40Gal Planted- 10Gal RCS Planted
6 Julii corys, 1m Furcata Rainbows, 2f 1m Bose Rainbow, 2 mystery snail