MONTPELIER, Vt. — A type of algae called “rock snot” that was thought to be an invasive species in the Northeast is actually native to the northern United States, researchers have concluded.

Mary Russ, executive director of the White River Partnership, holds a rock covered with the aquatic algae Didymosphenia geminata, known as didymo or rock snot, in the White River in Stockbridge. The ...