The Collaborative International Dictionary
blue-green alga \blue"-green al"ga\ pl. blue"-green al"gae any of a group of photosynthetic microorganisms possessing characteristics of both bacteria and plants. When classed as bacteria, they are assigned to the Cyanobacteria; when classed as plants, they are assigned to the Cyanophyta. They are now known to be prokaryotic, and are usually called cyanobacteria in technical contexts. See cyanobacterium.
Wiktionary
n. Any member of the cyanobacteria; a primitive photosynthetic organism similar to bacteria which is thought to have generated the first oxygen in the Earth's atmosphere.
Usage examples of "blue-green alga".
Third: the ship's colony of Chlorella, the blue-green alga housed in many miles of transparent plastic tubing in what used to be the hold of the Argo, irradiated by fluorescent lamps and constantly in circulation in the nutrient bath in which it grew.