If your leather sofa or chair has been exposed to water or moisture, mold or mildew can begin to grow on it. If you notice this, it is important to remove the mold or mildew as quickly as possible. Both can dry your leather sofa out, causing it to crack. Mold and mildew can also stain your leather sofa. Follow these steps to properly remove mold or mildew from your leather sofa or chair.