How is it "grossly incorrect"? Using both taxonomic and size classification, is it not accurate to refer to a blue-green alga as a "microbe" or "microorganism." [1]

A microbe (or microorganism) is generally defined as an organism that is microscopic—too small to be seen clearly by the naked eye. Blue-green algae fit this definition as they are single-celled or form microscopic colonies.

The scientific name for blue-green algae is cyanobacteria, which are technically a type of bacteria, universally classified as microbes. [2] They are prokaryotes (lacking a nucleus and other membrane-bound organelles), and the two prokaryotic domains of life (Bacteria and Archaea) are composed entirely of microbes.

[1] https://pmc.ncbi.nlm.nih.gov/articles/PMC9025173/

[2] https://doh.wa.gov/community-and-environment/contaminants/bl...