Algae and mosses grow in an acidic and moist environment. Orchids like to dry out between waterings while algae and mosses like to be constantly wet all the time. Whenever you water your orchids remember to water them thoroughly once or twice a day as climatic conditions demand rather than several halfhearted mistings spaced out several times during the day. If this doesn\'t solve your problem especially during the monsoons, then you can top dress with Dolomite or Calcium Carbonate(Oyster Shell). Remember that Oyster Shell generates a lot of heat and is not to be used during hot weather. This raises the pH making it impossible for the algae and mosses to grow. Never use bleaching powder to kill algae as it can also be detrimental to your orchids.