Is plankton a microscopic organism

Although algae range from single-celled organisms to multicellular organisms, if they are both multicellular and marine, and are easily seen by the naked eye, they are generally called seaweeds. Single-celled or few-celled organisms are not usually called seaweeds. Seaweeds themselves have many forms, including those that appear as if they are terrestrial plants with leaves and stems, looking like moss, mushrooms, leaf lettuce, or even a palm tree. Some are quite large: the multicellular giant kelp reaches 60 meters in length.