Green algae is made up of a diverse group of single-cell organisms that live mostly in freshwater, such as ponds and ditches. These microscopic green algae come in various shapes and sizes. They thrive in bodies of water that have few or no aquatic life but are filled with nutrients and exposed to sunlight.
Green algae actually provides many ecological benefits. For one, it is a major source of oxygen on the planet. These microscopic organisms are also the main food source for aquatic animals, such as small fishes and shrimp. Just observe your garden pond fish carefully and you will see that they eat the green algae growing on the sides of the pond or the surfaces of underwater rocks.
The large types of green algae, meanwhile, are not only a food source to some underwater animals. They also serve as a shelter to invertebrates and small fishes.
When Green Algae Become Harmful
The only time that green algae become harmful is when there is an algae bloom, or overproduction of algae in the pond system. Overabundance of some types of green algae can choke the production of oxygen and other vital processes such that they can actually cause death to the fishes in the pond. So, while algae growth is normal, if your water becomes so cloudy that light can't filter through, you may have reached the level of, "Too much of a good thing."