Hi redwhite, Welcome to All Things Plants!
It sounds like your terrarium needs a bit of aeration. Can you remove the lid for @ 24 hours or so, or perhaps leave the lid ajar for awhile to see if allowing the moisture to escape makes a difference? I've had terrariums of different shapes and sized for many years and the ones I've had trouble with mold or fungus growing were because I had not removed the lids in awhile and too much moisture built up inside the terrarium. I've learned to remove the lids once in awhile if moisture begins to fog up the glass and sometimes I just push the lid aside a tad and leave it for a few weeks to allow for more air circulation. Do you water the plants in your glass terrarium? If it's completely closed it won't need water very often if at all. Closed terrariums are very humid environments and it may be the moss just needs more fresh air than it's receiving.
I'm really surprised your succulent (jade) is surviving in a closed terrarium; I'd think terrarium conditions would be entirely too moist for succulent type plants.
I hope others will pop in here with suggestions and advice. Meanwhile ... again,
Lin
