Mildew is the common name for various molds, simple plants that are classed as fungi. It tends to have a fuzzy appearance and may be white, rust, green, or black, the latter being more common. Because it is a living organism that grows with warmth, humidity, and nutrients, keeping materials dry is key to preventing mildew. Removal efforts are not enough to prevent mildew. The source of moisture must be identifyed and corrected to prevent future growth. Refer to the Preventing Mildew in Your Home (pdf) publication for more information