Blue-green alga and Cyanobacteria

Cyanobacteria and blue-green alga are semantically related. In some cases you can use "Cyanobacteria" instead a noun phrase "Blue-green alga".

Cyanobacteria noun - Predominantly photosynthetic prokaryotic organisms containing a blue pigment in addition to chlorophyll; occur singly or in colonies in diverse habitats; important as phytoplankton.

Blue-green alga and cyanobacteria are semantically related. Sometimes you can replace phrase "Blue-green alga" with "Cyanobacteria", this terms are similar.