Green Algae is believed to be the ancestor of all plants. Although they share many characteristics with plants, like chlorophyll a and b, starch and cell walls made of cellulose, they are not considered plants.
Some green alga are unicellular, while others, like volvox, is colonial.
Then some, like spirogyra, are multicellular. Brown is a multicellular seaweed. Members of this phylum include sargassum and kelp. They are related to diatoms. Some, like kelp, are keystone species.