The answer, from the US Geological Survey’s Facebook page, is an algal bloom in Lake Erie, as photographed by Landsat:
In late September, Earth-observing satellites documented a large algal bloom in Lake Erie, which coated much of the water with a bright green skin-like slime. This goo, called cyanobacteria, is made up of microorganisms similar to algae and bacteria. When excess nutrients build up in the lake, cyanobacteria multiply rapidly, creating harmful algal blooms that can threaten human and aquatic ecosystem health and cause major economic damage. (Read more about these blooms at https://go.usa.gov/xn29R.)
Remote sensing satellites such as Landsat, which is operated by the #USGS and built by NASA – National Aeronautics and Space Administration, continuously search for green cyanobacteria, and can help managers prioritize at-risk waterbodies: https://go.usa.gov/xn29E.