It's an algae bloom - NOT AN OMEN.
It's caused by excessive deposits of nitrate, typically from chemical fertilizer or manures.
Either there was some nasty run-off or leaching from some farms upstream, or somebody dumped sewage.
So until somebody draws out a gallon of pure blood out of the water, pay this stuff no mind. Otherwise you're displaying your ignorance to anyone who works with or lives near a significant body of water.