Most species of algae are not harmful, but certain types of algae, blooming in excessive amounts can cause severe harm to human health, aquatic ecosystems, and local economies. These harmful algal blooms (HABs) are usually associated with algae that produce unhealthy toxins.

Sometimes HABs are referred to as red tides, but this can be misleading since they may be different colors. Also, even though they are classified as bacteria, cyanobacteria exhibit the characteristics of algae and are associated with HABs. EPA researchers are looking for ways to eliminate or reduce the negative effects of HABs on human health and the environment.  See More at USEPA via https://www.epa.gov/water-research/harmful-algal-blooms-cyanobacteria