Hi Pinkrose,
to All Things Plants from me too!
Leaving the lid off of your lovely terrarium will help dry out the soil and yes, I'd try to remove as much as that stringy mold as is possible without damaging the plants. If you aren't up to completely emptying, cleaning and replanting your terrarium I'd say leave the lid off for a week or more to let the soil completely dry out and see if perhaps that takes care of the problem.
At one time I had a couple of terrariums that look very similar to yours and moisture would build up so quickly that I never had to water and I still had to remove the lids for days or a week at a time to allow air circulation and to let it dry out.
Here's some information you might want to read regarding mold in terrariums:
http://www.stormthecastle.com/...
