Question

If blue green algae is different from algae why it is named as algae

Answer 1

Because they are photosynthetic and aquatic, cyanobacteria are often called "blue-green algae". ... Cyanobacteria are relatives of the bacteria, not eukaryotes, and it is only the chloroplast in eukaryotic algae to which the cyanobacteria are related.

Answer 2

It is cyanobacteria of Kingdom Monera with phycocyanin pogment in them. They have several prokaryotic features which make them primitive algae.

Blue green algae was once considered belonging to algae of plants. Soon it is found that it has several primitive prokaryotic characters and hence it was put under Monera naming Cyanobacteria.