Mold will grow if the humidor is too moist. You must always be conscious of how much water you are adding to the humidification device. Mold will not grow quickly, so there are 2 signs that you can look for that will indicate that you are heading in a bad direction. The first indicator is damp, soggy cigars. These will be hard to smoke, and may even hiss or burst as you smoke them. The second sign is a foul, musty odor that greets you when you open the humidor. If you notice either of these symptoms you should cut back on the amount of water you are adding to the element, regardless of what your hygrometer (if you have one) says. It is probably not calibrated and is giving a false reading. If you spot mold on a cigar remove it immediately from the humidor, as it will spread to others.