Microcystis

Microcystis is a variety of blue-green algae that typically appears as thick green scum on the surface of nutrient-rich bodies of water. The blue-green algae (Cyanobacteria) blooms are a natural phenomenon in nutrient-rich lakes, reservoirs, and slow moving bodies of water around the world. 

Microcystis: Toxic Blue-Green Algae Fact Sheet (PDF)