Didymo (Didymospenia geminata) Didymo, or “rock snot”, is native to cold, high altitude waters of Europe and Asia. Unfortunately, it has been introduced to North America and has recently spread to many waters here. Didymo has been found in Vermont, New Hampshire, New York, Virginia, West Virginia, Tennessee, Colorado, Arkansas, Wyoming, Utah, Idaho, Montana, Washington State, North Dakota, South Dakota Alaska, and British Columbia. In its native habitat, didymo’s growth is in balance with the surrounding ecosystem and it grows in small colonies. However, in non-native habitats, it grows into huge mats that can cover the width of the river bed.
Although it is a single-celled organism, didymo grows in algae-covered stalks, often creating thick mats that covers the width of a river bed. These mats change the river substrate and other aquatic conditions, causing declines to invertebrate populations on which trout feed. The mats are so thick they crowd out native plants that normally provide shelter to invertebrates and small fish. Didymo infested waters have lower abundances of adult trout and higher abundances of juvenile trout. As fish get older, they become hard pressed to find enough food in the didymo-altered ecosystem.
Didymo is easily spread to uncontaminated waterways. If even one cell of didymo is transported to clean waters, it can become contaminated with the species. It is easily spread by infected recreational equipment. Felt-soled waders and shoes are especially problematic as it is difficult to remove the plant material from the felt. Various state agencies are monitoring didymo populations and working to educate anglers and other water users about the threat.
"On the Riverfront, Warnings of an Aggressive Algae" 
New York Times article by Tim Wacker, published August 5, 2007