Algae is the term for several groups of simple living organisms that capture light and process it through photosynthesis to create the energy they need to live and grow. They can occur in most habitats but are usually found in damp places or bodies of water. Phytoplankton, living in oceans and lakes is the basis for most marine food chains. Some are used as human food or harvested for useful substances such as agar or fertilizer.
Red Algae is different from the other species; it is classified in Phylum Rhodophyta, not Phylum Phaeophyta like the others. Red Algae is multicellular and live warm marine environments rather than cold. It is actually rather similar to coral.
Reference: Exploring Creation with Biology Wile and Durnell