This is a fun question. I've encountered this figure vaguely and I believe
he's tied in with the Lord of the Animals or Horned God of Celtic lore and
many other cultures (almost everyone had a Lord or Lady of the Animals).
I read a children's retelling of an English legend of the Green Man that
it was a lord lost on the hunt who had to survive on his own for the first
time in his life and learned gentleness thereby--and when he rejoined his
noble life another Green Man came to take his place. The story also
implied that in this medieval time the legend was used to frighten children
and keep them from wandering too far into the woods from the village (the
Green Man'll get you if you don't watch out...).
Both mundane and religious aspects may be fact; legends are used for all
sorts of purposes.
I'd like to hear more about this.