I have never used it but I believe a little vinnegar will remove mosses and algae from pots. I seem to remember that a potassium permanganate solution is the traditional cure. I think tar oils would also work (armillitox, or winter tar wash for fruit trees), perhaps sulfer dust might do the job too? . Mosses will grow to top of magnesium carbonate and calcium carbonate rocks and walls, inches thick, in Derbyshire England, (alkaline rocks). Mosses and algae will also grow well on the acid grit-stone, and in enclosed or wet enviroments on 6 millimeter or larger gravel- including granite. As a foot note for those in drier climates - one of the jobs of a top dressing is to prevent "puddling" (compacting of the surface) of the growing medium by rain drops. I concur with Mark that moss makes an excellent top dressing and growing medium for some plants, for example Pleione or Trillium. I usually repot or else replace the compost and top dressing from the bulbs (plant roots in the case of non bulbs) upwards. This helps remove other weeds too. Peter (UK)