Question

why are blue green algae included under Monera and not under plantae

Answer 1

this is because algae may be able to produce their own food but they are mainly prokaryotic in nature. It means that their cell organelles and nucleus lack membrane. their blue green colour is due to the pigment phycocyanin

Answer 2

Dear, blue green algae is otherwise cyanobacteria. Its primitive and does not have any basic features that the kingdom plantae posses. For e. g it has a cell wall but it is non cellulosic but the plants cell wall is made of cellulose.