Lots of great ideas. What is that from anyway?
I would use that to represent a manifestation of the Green itself on the Prime Material Plane, taking control of a forest to speak to the PCs. (The Green is taken from Monte Cook's Arcana Evolved, where it's the force of life and energy, opposed by the Dark.)
Alternatively, it's the death site of the greatest ranger ever known, who was so steeped in magic that he lives on in the moss, his mind ranging over the trees and rocks and brush, guarding the entryway to the great Forest Primeval.