Rock snot, also known as didymo, is an invasive algae that occurs in cold rivers and streams of Northeastern North America. Rock snot has been found in16 American states and Quebec, Canada. This invasive form of algae covers river and stream bottoms with dense mats and long strands.

Rock snot can be distinguished from other aquatic algae species by a number of characteristics:

 - coloration is brownish to white but not green

 - texture is similar wet wool

 - attaches firmly to rocks and stones

According to biologists, rock snot does not pose significant risks to humans but can have negative impacts on rivers and streams.

When rock snot blooms occur, the resulting mats are thought to be harmful to streams in a number of ways:

 - mats can entrap or block minnows, aquatic insects, crayfish, and other stream life such as caddis fly, may fly, stone fly, minnows

 - blooms make fly fishing difficult or impossible

 - rock snot is slippery