The presence of slime mold is more of a curiosity than a problem. Slime molds are primitive organisms that are considered fungi. They feed on decaying organic manner and other organisms in the thatch layer and soil.
Slime molds often appear after a warm summer rain. At first a slimy growth, called the plasmodium, appears. This slimy growth dries into a powdery mass of spore-bearing structures that coat grass blades. Slime molds usually last one to two weeks and often are observed in the same spot year after year. Typically 4-6 inches patches of the fungus are formed. Although there are many species of slime mold on turf, Physarum cinereum seems to be the most common.
Control measures are not usually necessary. If desired, slime mold can be removing by raking, sweeping with a broom, spraying with stream of water, or mowing.
See this article for more information on slime molds.