Mold is usually a dead giveaway that the terrarium has excess of moisture. If you experience white mold in your terrarium, chances are the water inside it might be too abundant. First thing you need to do is removing the lid off for a few days to promote air circulation and reduce humidity, following you can use a cotton swab dipped in water to remove any mold covering the plants, rocks and decorations. Don�t use the same cotton swab twice so you won�t spread the mold around. If you find it difficult to remove mold from some plants, you might want to just trim or remove the plants from the terrarium.