CASSVILLE, Mo. (AP) — Missouri conservationists and anglers are working together to prevent an algae called didymo — its nickname is “rock snot” — from spreading into the state’s waterways.

Kansas fly fisherman Paul Niegsch frequents Missouri’s trout streams and says he hadn’t heard of the algae until seeing signs posted last year by the Department of Conservation.

The non-native invasive species has expanded in recent years to 18 states, including a 13-mile stretch of the White River in neighboring Arkansas near the Missouri border.

Didymo is a single-celled algae that blooms in freshwater rivers and streams with consistently cold water temperatures and just the right pH level. Conservation officials say the state’s trout streams provide ideal conditions for didymo to flourish.