Looking vulnerable, the green stuff in picture is alive and on the edge. Come to think of it, all life is on some edge, sometimes unnoticed when the comfort zone appears wide, the take on reality is that all is well and safe! The thin green layer on recently dried earth is likely one of many kinds of green molds freely occurring in nature and sometimes bothering people at home. Here it is merely a thin coat on dried mud, sometimes seen in the veld.
A mold is a type of fungus, a plant-like organism that does not photosynthesise, obtaining its nourishment directly from the environment. Molds appear in a various colours including green. The colouring is comparable to flowers and fruit in plants; the rest of the fungus consists of a network of thin, transparent filaments that are less visible or conspicuous than what is seen here.
These mold fungi grow on many surfaces, including soil, damp walls and decaying food. They reproduce by minute spores that are surprisingly resilient to temperature extremes and moisture or drought. In nature molds serve in numerous natural life cycles breaking down organic matter.
They serve human endeavours in the manufacture of certain foods, antibiotics and drugs (Wikipedia; https://www.allthingsnature.org).