This icky sounding aquatic invader threatens tourism, fisheries and even hydropower facilities that depend on our streams and rivers. Why? Rock snot, aka didymo (Didymosphaenia geminata), is a one-celled alga or diatom. Each tiny didymo grows a gelatin-like stalk it uses to attach itself to rocks, plants and just about any available surface under the water. Didymos reproduce by dividing and so do their stalks, eventually creating a branching network that feels like wet cotton or wool. Thus,...