noun
powdered green laver
Only when written 青粉
Dried and ground green seaweed (similar to aonori). The kanji 青粉 literally means 'blue/green powder'.
See also: 青海苔
青粉をふりかけて食べると風味が増す。
Sprinkling powdered green laver on food enhances the flavor.
noun
blue-green algae; cyanobacteria
Refers to the microorganisms themselves, especially Microcystis. Usually written in katakana.
アオコが発生すると水が緑色に濁る。
When blue-green algae bloom, the water turns green and murky.
noun
algal bloom; water bloom
The phenomenon of excessive growth of algae or cyanobacteria in water bodies. Often used interchangeably with 水の華.
See also: 水の華
今年の夏は湖で大規模なアオコが発生した。
This summer, a large-scale algal bloom occurred in the lake.
From 青 (ao, 'blue/green') + 粉 (ko, 'powder') for the powdered seaweed sense. The algae sense is an extension of the color and appearance, but the exact derivation is uncertain.