The problem is moss growing in turf. See this publication for methods of managing moss in lawns!
Controlling Moss & Algae in Turf
Occasionally, turfgrass areas begin to thin out and moss and algae start to form. These primitive plants develop because conditions for growing dense, healthy turf have declined.
Mosses are branched, threadlike green plants that form a tangled, thick mat over the soil. Algae are thread-like green plants that form a dense, green scum over the soil surface. Neither moss nor algae are thought to be parasitic. Both are spread by wind-blown spores and can form crusts on the soil surface that reduce air and water movement into the soil.
Factors that favor moss and algae development include wet, humid conditions and compacted soils with thin turf. Moss is more common in shady areas with infertile, acidic soils and excessive thatch; algae is more common in full sun conditions and fertile soils.