Answered
what is it?

That green button doesn't move and doesn't disappear.

That green button doesn't move and doesn't disappear.
On Mac OS this green icon usually appears when you drag and drop objects, text, folders, files etc. between different applications or between different documents in one application. It indicates that the dragged objects can now be dropped.
If the icon is there permanently, I suppose it is just a harmless refresh/redraw issue on screen.
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