In a shaded, moist area the most likely organisms you might encounter are mosses, alga or slime molds. Mosses are green, and the plant has some identifiable structure. Alga species come in many colors from black to gray to green to yellow or red. Some algae look like velvet, while others appear as an amorphous, slimy, shiny mass. Slime molds are usually very periodic in their appearance, being visible for a short time a few times per year. Usually, all these plants are found where there is more or less continuous moisture, shady conditions, and poor (low nutrient) soils. Fortunately, all three of these primitive organisms can be controlled in the same way. One possibility is to dry up the area or at least allow the area to dry out between irrigations. Another possibility is to fertilize the affected area. Cover the area with one half to three quarters inch of composted manure. This increase in fertility will discourage moss and algae growth. A third possibility is to reduce the shade and increase air circulation. Finally, spray the area with a copper fungicide. Copper compounds are toxic to mosses, alga and slime molds. A periodic spray of the area should eliminate the problem. Use all copper materials according to container label instructions.