The Didymo, AKA Didymospheria geminata
Often called "rock snot", it is found on rocks in streams and rivers, where it forms thick, brown mats to cover up the bottoms of water bodies.
It is brown, sometimes a lighter shade, with a close resemblance to mucus, hence its nickname. The actual size of didymo can vary, sometimes it is only small patches whereas other times it can cover the entire rock.
It is now found in Scotland, Poland, the Northwestern Region of the U.S. as well as Quebec and British Columbia
Spreads by clinging to the bottoms of boats and insides of gear, and when it is brought to another body of water (a river, pond, etc) it will cling to rocks and spread
The U.S. vs Didymo
Its main threat is that it will cover up the bottoms of streams and choke out organisms that live in the streams, thus removing food sources to fish and other aquatic life that live there
A useful way of dealing with the didymo is preventing anyone from wearing felt-soled wading boots, as they act as a vessel for didymo