If you do see mold remove it. Open the terrarium to let it dry out a little. Remove any dead or dying plant tissue as it appears, as these can start to decompose in the warm, moist environment.

I didn't notice the night I got home because I was exhausted, but the next morning, I went to check on the terrarium to find it in a terrible state. The roots were rotting and the plants dying and ...

A terrarium is an indoor plant decoration in ... so the roots might rot. This results in the plant dying. "It is better to be less wet than too wet because plants that are root-rotted can ...