I wonder if the mold/fungus could be in the soil; I've heard of bagged potting mix containing mold spores. I know that mold thrives on moisture so if you could take the lid off of the terrarium and perhaps have a fan circulating air, maybe the mold would die on its own. I'd probably break down the terrarium and build it anew with fresh soil and wash the soil from the roots of any of the current plants in case spores are attached to them which would just cause a new growth of the mold.
I did a site search for terrariums and came up with one article and a few different threads where terrariums were discussed. I don't have time to go through them right now as I need to finish cooking our dinner but if you have time you might find some useful information regarding terrariums.
http://garden.org/search/index...
