Mold and mildew are terms that are used generally to describe growths of fungi on various surfaces. Mildew also is a scientific term that describes a type of plant disease. In common usage, the difference between mold and mildew usually is in their appearance and the surfaces on which they are growing. Mold is often thicker and black, green, red or blue in color, and mildew usually is lighter, powdery and gray or white.