Algae come in mostly red and green. Most freshwater algae are green. This is the stuff that makes water turn green when it is becoming stagnant. Algae tend to be long filaments of plant material although some can exist as single cells.
Green algae are producers. They catch the sunlight that penetrates the water while they absorb nutrients from the water and produce sugar and oxygen just the way their larger land loving cousins do. Algae can become so dense that they can clog pipes and can be scooped up almost like seaweed.
Most fish eat algae as at least part of their diet. Other single cell and simple organisms depend on algae for protection and food. The oxygen produced by algae supplements the oxygen absorbed at the surface of the water to keep a steady supply for water-breathing creatures.
Excess oxygen produced by the huge quantities of algae and other aquatic flora finds its way into the atmosphere to help maintain the 20% or so of atmospheric oxygen that land creatures, including humans, need to live.
Some blue-green algae team up in a symbiotic relationship with a fungus to make lichen. Lichen is an important food source for some smaller animals and insects. It also helps to erode rock by secreting a weak carbonic acid. This makes a small quantity of soil to allow moss to grow. Eventually, the process leads to forest or prairie.