The term 'blue-green plant' is a reference to an ALGA, which is a type of organism that is classified as a protist. Algae are typically found in aquatic environments and can range in size from microscopic single-celled organisms to large seaweeds. They are known for their ability to photosynthesize, meaning they use sunlight to produce energy and nutrients. Algae are also important sources of oxygen and food for many aquatic species.