Filamentous Algae (Spirogyra spp.; Order Zygnematales) are single algae cells that grow in water and form long threads that intertwine to form a mat resembling wet green wool. Like higher plants, Filamentous algae uses chlorophyll to photosynthesize.

Filamentous algae starts growing on the bottom of a shallow water body attached to rocks or other aquatic plants. In time, the mats become thick and dense and trap oxygen and other gases that cause the mat to float to the surface of the water, sometimes forming large mats.

Filamentous algae has no known direct food value to wildlife, but protozoans, aquatic insects, fish, and tadpoles eat it, making it an important part of aquatic ecosystems.

There are several genera of filamentous algae. Around Las Vegas, there are two common genera. Spirogyra (common names include water silk, mermaid's tresses, and blanket weed) is a genus of filamentous green algae in the order Zygnematales, and Cladophoropsis is a genus of green algae in the family Boodleaceae. Spirogyra feels like snot, while Cladophoropsis feels rough.