Didymo is a species of diatom that grows in warm and shallow water. If it overgrows, it can form large mats on the bottom of lakes, rivers and streams. It is not considered a significant human health risk, but it can affect stream habitats and sources of food for fish and make recreational activities unpleasant. It is considered a nuisance organism or invasive species. The microscopic alga can be spread in a single drop of water.

Rock snot is a type of slimy, yellow-brown, freshwater diatomatious algae that, like a cold, has spread quickly from a small area on Vancouver Island.

t's now a "global invasive species," scientists say, spreading rapidly within B.C. and to locations such as Iceland, New Zealand and across North America — including Alberta, Quebec and New Brunswick.

There's also an economic and recreational impact. Didymo can slough off the bottom of the river during high water flow. When it dries on the river banks it often looks like dried-up toilet paper, causing people to think there's a sewage problem in the river. It greatly degrades the recreational experience.