Blue-greens are very primitive organisms that are not really algae. They photosynthesize like algae, but they are actually bacteria.  Scientists refer to them as “cyanobacteria” to acknowledge that they are bacteria. “Cyan” means “blue”, which refers to the fact  that these organisms often appear blue-green in color. Where do the blue-green algae live? The blue-greens split into two major groups, the planktonics and the mat-formers. The planktonic blue-greens are microscopic and cause the typical pea-soup green color to water. The most common of the planktonic blue-greens are Anabaena, Aphanizomenon, and Microcystis (commonly referred to as Annie, Fannie, and Mike!). In addition to causing water to turn green, they can rise to the surface of calm or static waters and form surface scums. This yellowish-green scum formation is typical of most planktonic blue-greens. The formation of surface scums tends to block the light to the other types of algae and aquatic plants that live deeper in the water column. By shading out their competitors, blue-greens can completely dominate a body of water - Source

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Planktonic blue-green algae: Production, sedimentation, and decomposition in Lake Mendota. Source