They are microscopic and single celled. Diatoms look like two shells that fit together. They are microscopic and single celled. Algae are photosynthetic creatures. They are neither plant, animal or fungi.
Many algae are single celled, however some species are multicellular.
Many, but not all of red and brown algae are multicellular. Red and brown algae are common in marine environments. Some green algae are multicellular. The scum found on ponds are algae. They are the green hairy growth on things found underwater objects.