Moss and Algae
Moss or algae are found in turf areas because conditions are not good for growing dense, healthy turf. Mosses are small plants that have a mass of fine stems. Algae are thread-like green plants that form a thin, dense, green scum over the soil surface. Neither moss nor algae are thought to be parasitic to turfgrasses. The green scum formed by algae is relatively impermeable, and once it dries out, it forms a tough, black crust.
Algae will appear more in wet or humid full-sun locations and compacted, water-logged fertile soils with thin, weak turf. Moss tends to take hold in wet or humid shady conditions; acidic, infertile, poorly drained, water-logged soils; or areas with excessive thatch and thin, weak turf.
If you have an abundance of either moss or algae in an area, you have two choices. First, don't try to grow grass in these areas, because the conditions are probably not right for it - and you’ll end up with a constant struggle for healthy grass. Second, if you really want turf in a mossy area, you must change the conditions. The changes necessary may include the following:
1.) Have the soil tested to determine proper lime and fertilizer needed for best soil fertility.
2.) Improve drainage - either by creating a contour in the soil or by using tile.
3.) Increase light penetration and air circulation - trim low branches on trees and shrubs. In some cases, removal of the least-desirable trees may be justified. Using a shade-tolerant grass will help.
4.) Cultivate compacted soils - aerification with a machine that removes plugs of soil will help to reduce compaction.
5.) Avoid excessive irrigation - keeping the surface moist will only increase problems.
Product control of moss and algae is temporary, and the problem will recur unless growing conditions are improved. Once you improve the conditions, sodding or seeding are the recommended means of establishing turf under heavily shaded conditions.