It's not algae. It's cyanobacteria.
Big difference.
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Cyanobacteria /saɪˌænoʊbækˈtɪəriə/, also known as Cyanophyta, are a phylum of bacteria that obtain their energy through photosynthesis,[4] and are the only photosynthetic prokaryotes able to produce oxygen.[5] The name "cyanobacteria" comes from the color of the bacteria (Greek: κυανός, translit. kyanós, lit. 'blue').[6][7] Cyanobacteria, which are prokaryotes, are also called "blue-green algae",[4][8] though the term "algae" in modern usage is restricted to eukaryotes.[9]