You need to decide whether its slime mould or algae - treatment for the latter is mosskiller, and improving turf conditions, because it usually occurs either on thin, sparse grass which is poorly nourished and not maintained, or on thick turf which is cut very short frequently and not nourished and maintained. Slime mould grows on the stems of the grass and appears to dissolve when wet, and the only treatment for that is washing off with a jet of water from the hose.