Given the temperature difference between the terrarium and the room where it's placed, you all notice some fog appearing on the walls of the container due to condensation. This is completely normal, and it is part of the small scale water cycle taking place inside the mini ecosystem. If instead, you see water drops running down the walls of the container, chances are there's too much moisture inside. Proceed to take off the lid for a couple hours to let some of the moisture to evaporate and put it back on.