Seaweed is a term applied to multicellular, marine algae which are large enough to be seen by the eye unaided. Some can grow to up to 60 metres in length. Seaweeds include members of the red, brown and green algae.They are members of the kingdom Protista meaning they are not Plants.They do not have the vascular system (internal transport system) of plants and do not have roots, stems, leaves and flowers or cones.lue-green algae are not marine algae. They are in a group called cyanobacteria and are more closely related to bacteria. Some cyanobacteria form brown, green, red or purple tufts on coral reefs. To survive seaweeds need salty or brackish water, sunlight and a surface to attach themselves to.