Blue Green Algae also called as Cyanobacteria are one of the most successful organisms on this planet. Initially, they were classified Plantae, but later they were moved to the Monera Kingdom.
We can easily distinguish a living animal from a living plant in our surroundings. So, to keep it as simple as it was, Carolus Linnaeus classified all the organisms of the world into two Kingdoms i.e. Plantae and Animalia. But the grouping of such diverse organisms into only two kingdoms was not satisfactory to many biologists of that time.