Slime Moulds are most common during prolonged wet periods during the summer months. Initially they are slimy to touch hence the name, they later dry to form spores that look like ash, these spores are attached to the leaf blades on the grass. The good news about slime moulds is that they do not affect living plants, they live off dead and decaying plants.
There is not a recommended control measure as this disease is not harmful to grass, The only problem is they can sometimes cause the grass to turn yellow, as they restrict sunlight getting to the plant. Slime moulds can be simply brushed off the leaf or the grass with a brush or rake, however they will disappear once the weather changes to drier conditions.