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. Algae releases oxygen into the water as it manufactures it food. Algae forms the broad base on which the food pyramids in ponds and lakes is built. In manufacturing food, algae release oxygen, increasing the amount dissolved in the water.