Fungal/mold spores are ubiquitous -- merely quarantining plants will not prevent fungus in terrariums. Spores on the rocks, plants, etc. or mycelium in the media can remain dormant or otherwise undetectable until optimal conditions occur for rapid growth. For that matter, fungal spores could easily have been introduced into your terr as you were putting it together since spores are frequently airborne.

Depending upon the type of fungi, the plants may or may not be affected. No way to tell just by looking at the fungi, you'll have to simply see how the plants fare.

Fungi generally like moist stagnant conditions. Because many folks like to keep their terrs closed up to minimize watering needs, molds are common.

You photo, unfortunately, is rather unclear (always a challenge taking a pic of something in a jar). The foggy areas are what I assume you are talking about look like moisture condensation in the pic. It might help if you allow the media to dry out somewhat -- ie. if you have a lid on the jar, remove it. A cautionary note: always err on the stingy side when it comes to watering -- much easier to add a sip here or there than to fix an overwatered terr.

If you haven't already done so, a perusal of the FAQ for this forum may be useful.

thanks for your response. my terrarium does have an opening in the the top. it is a wine jug, so the opening is rather small (just the bottle opening) but i do not keep it plugged or closed off in anyway. It is difficult to take a picture of something in a jar, but the condensation you are referring to is actually the white mold. the picture is sideways, too, which adds to the difficulty. Basically, the mold has contained itself to that bottom layer where the rocks are. And since posting this question a few weeks ago, it has not changed at all. The plants continue to do great and the mold just seems to be happy hanging out under all the dirt. I have set up a small solar fan in the bottle opening which comes on at various times of day (whenever there is enough sun) in an attempt to dry the terrarium. I have not really noticed this helping much, if at all though. All in all though, the terrarium seems to be doing fairly well. I'm just hoping that mold stays where it is instead of coming up to the surface and wreaking havoc in the plants!