Watermeal

Watermeal (Wolffia spp.) is the world's smallest flowering plant — rootless fronds under 1 mm across that look like green grit or grass clippings on the water surface. Frequently mixed in with duckweed but much harder to control: watermeal slips through most filtration, most herbicides only stress it, and dense coverage can survive winter under ice. Indicates extreme nutrient loading. The fix is the nutrient source, not the plant.