Q.What is this green stuff that has invaded my yard and what to do about it?
For some time now my whole yard front and back has been invaded by this green stuff (is it moss??) all over. At first I thought it was due to the humidity caused by the rains during the summer season, but even when it stopped raining and the sun is really hot it doesn’t go away. Is it killing my plants??
I see it on the trunk of my trees, on the trunk of the palms too, and on all the branches of my hedges. I have various types hedges, variegated arboricola, Gold-green dusted crotons, and others i don’t know their names. But my arboricola is losing leaves and so are the others, They just don’t seem as full as before even though we take care of them and fertilize them etc. Could this be the cause??

This is Lichen. It really is not harmful to your trees or plants but rather a result of growing conditions.
These links will help you.
https://www.gardeningknowhow.com/ornamental/fungus-lichen/treatment-tree-lichen.htm
https://www.gardeningknowhow.com/ornamental/fungus-lichen