In areas where there is not enough sunlight or air movement, mould, mildew and moss will start to grow. These slippery, unsightly areas are unattractive, and they are also unsafe. Non-flammable, Simple Green Outdoor Cleaner concentrate is a TOUGH, no nonsense way of dealing with large areas of mould and moss. Simple Green Outdoor Cleaner leaves a mild residue that will eliminate the fast re-growth of moss and mould.