Actually it probably is a moisture problem. Too moist. Keep the top ajar and let it dry out some or siphon some water out. Standing water isn't good for any plants.

Moss is very durable and hard to kill. The two main ways to kill it is by HEAT Sterilization and Water Rot! Letting the moss dry out every so often will help it. Fertilizers are also bad news. The salt in them isn't helpful. If the PH of the Terrariums isn't about 6.5 most moss isn't happy. Sphagnum peat moss is a great item to include in the soil. As it breaks down it helps keep the PH low and is also a great base for most plants. Moss likes air flow. Try cycling the with the lids off for few days every so often or putting (three) small pieces of one sided foam tape(space out evenly) on the lids sealing edge. It would allow the lid to close but allow air in. Also broken pieces of clay pots make great drainage for extra water to drain down but also wick up as needed. Hope this helps.

I agree, I think stagnant water is the problem. Also, what water are you using? Don't use tap water for anything in your beautiful terrariums, especially mosses and ferns. I've found that even the slightest bit of tap water kills my moss. Tap water chemically burns it. And my ferns will pinch at the stems and die. Use distilled or rain water. Just like houseplants, water in your terrariums should be less in the winter (Winter is resting time, so less photosynthesis and heat/evaporation to help circulate the water and produce condensation. So your getting root rot.)

Again, I love your jars! Best of luck!