Wiktionary
n. A dense spread of algae on the surface of water
Wikipedia
An algal bloom is a rapid increase or accumulation in the population of algae in freshwater or marine water systems, and are recognized by the discoloration in the water from their pigments. Cyanobacteria blooms are often called blue-green algae. Blooms which can injure animals or the ecology are called " harmful algal blooms" (HAB), and can lead to fish die-offs, cities cutting off water to residents, or states having to close fisheries.
Usage examples of "algal bloom".
Each wore a reddish-pink halo of algal bloom in which fan-tailed sievers of bloated mien held court, arcing and diving like pale prima donnas in a perpetual alien pax de deux.
But the lake was choked with reeds and lilies and weeds, and on its surface he could see the sickly gray-green sheen of an algal bloom.
Suddenly they are banking over Manila Bay, which is marked with endless streaks of brilliant red--some kind of algal bloom.