The Green Man is often claimed as a Pagan symbol (even in church guidebooks these days). Presumably this is because a very small number of them appeared on Roman and Greek temples. But then there was a huge gap before they reappeared on churches in the Middle Ages.
It could be argued that sticking leaves on something is a Pagan impulse, because Paganism "is" nature spirituality. Well, neoPaganism is focused on nature, but ancient paganisms were generally about propitiating hostile nature spirits.
The figure of the Green Man seems to me to be a little bit Sufi - it reminds me of Al-Khidr, the Green One who is a friend of the soul. On the other hand, it could be a little bit Hindu, since the heart chakra is often represented as green. Or it could be a little bit Christian, since Christ represents Tifereth (the equivalent of the heart chakra in Kabbalah) and is certainly the friend of the soul. Or the Green Man might represent the greening of the soul in response to Divine inspiration. The vines or branches coming out of the Green Man's mouth might represent the truth being spoken (and there is a saying that the Holy Spirit speaks through everyone who speaks the truth, and She certainly speaks through Nature). Or it may represent a renewal of spirituality.
So let's not be too quick to claim that the Green Man is an exclusively Pagan symbol - it has resonance for people of all religions and spiritual paths. Especially now, when we all need to get into right relationship with Nature.