Question

microorganisms are called 'environment cleaners' explain

Answer 1

they feed on dead and decaying organisms and hence turn them into manures

Answer 2

The source of food for the microorganisms or the microbes is the decaying matters. They just feed on the decaying matters, or dead remains of plants, animals and humans, and turn them to manure which can be used by the plants as a source of nutrition. Thus, they clean the environment, and hence, they are called as "environment cleaners".