Sounds like you have it too wet! What plants are you growing? How often are you watering? Terreriums don't need much water at all! Do you have it set up with false drainage, (rocks and sand at the bottom under the soil to act as a place for the water to go so it's not just sitting in the soil)! How are the plants doing?
Have you noticed if the mold is on dead leaves or actually on the soil?

this thread might help

terrarium info