Cyanobacteria which is also blue green algae is harmful to humans because Cyanobacteria can produce deadly toxins that can taint drinking water and water that people swim or play in.
If a lake or other water source has blue green algae "Cyanobacteria" you should avoid drinking the water or even swimming or coming into contact with the Cyanobacteria tainted water because it could prove deadly to you or someone else who comes into contact with it or drinks the water containing Cyanobacteria.