Algae grow fast when exposed to sunlight, for instance, when your Moccamaster is placed near a window. Moccamaster recommends treating an algae contamination with Clean Drop. To use Clean Drop, fill half of the water container with hot (not boiling!) water and add a sachet of Clean Drop. Wait one hour, then pour out the water used for cleaning, any chunks of algae still left in the container can be cleaned with a paper or cloth towel. ,Rinse the container with clean water at least twice, before turning the machine on for a water-only cycle.