I would say that you DID overdo the watering.this is an easy fix. simply take the lid off, and dont water for a few days. african violets HATE having water standing on their leaves.the water gets trapped between the hairs on the leaves, and eventually molds over.
the plant in the back, other than beautiful, looks like a peperomia species. it may not be, though, im not sure. peperomia do pretty well in terrariums.

Sounds like you overwatered. That is one of the most dangerous things with a terr. With a closed terr especially, overwatering --even once -- is the kiss of death. Wet conditions plus no air movement equals mold city.

Take the lids off the jars and keep them out of direct sun but where there will be good air circulation. Better yet remove the plants and the media and start over. A sealed jar can be used BUT you must adjust the water content for the plant in question -- and that is not easy in most cases.

I currently have a 10 gal tank that is essentially sealed -- this one is basically a bog terr. I generally have to add water maybe once a year and have had this set up for over 10 years. Over time some plants have died out. Currently it houses a carpent of moss, a large volunteer fern, and a couple of dendrobium orchids which should have died years ago but refuse to do so. The media is wet and humidity is usually around 100%. Most plants would not survive this set up.

I also made some sealed terrs for friends and family. With av's it is essential that the media be just barely moist. It is far better to err on the side of caution and have the media too dry as opposed to too wet. If after sitting sealed for a day in bright indirect light there is a little condensation along the soil area you should be okay. If you have condensation appearing above the soil area you have too much and must remove the lid to let it dry out. If there is no condensation at all period, wait another day or two. If there is still no condensation you may add a few drops (literally) of water. Repeat condensation check.