Which term refers to blue-green algae?

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Blue-green algae are cyanobacteria, a group of photosynthetic bacteria that were historically classified with algae. They contain chlorophyll a along with phycobiliproteins, which give the characteristic blue-green color, and many can form blooms in water, sometimes producing toxins. In older classifications, they were placed in a class called Cyanophyceae. The other groups listed are true algae with different cellular structures and pigment combinations: green algae (Chlorophyta) have chlorophylls a and b, diatoms (Bacillariophyta) have silica cell walls, and red algae (Rhodophyta) rely on phycoerythrin for their color. So the term referring to blue-green algae is Cyanophyceae.