Green man masks are a staple seller on Merchants’ Row at any Pagan festival. I found a weather-resistant example at Beltania a couple of years ago, and now it hangs by the front door.
As Paul Kingsnorth writes in Aeon:
There are plenty of hypotheses [about his origin], and it depends on whom you talk to. Those inclined towards paganism like to claim that green men are relics of pre-Christian religions that have been incorporated into churches. I have heard, variously, that the green man represents the spirit of the greenwoods, the rebirth of nature, a rebellion against Christianity, or a symbol of the constancy of nature. Everybody who knows the green man has their favourite theory about what he is and why he is there.
But Kingsnorth goes on to offer a different origin story for the Green Man: that he is a symbol of ethno-political resistance to the Norman Conquest of 1066.
It would seem that if that is true, however, there would have to have been a cover story to tell to the Norman bishop in charge of the cathedral. And as one of the commenters points out, the Green Man figure is not unique to England. So maybe the nature-spirit reading of this figure still “has legs,” even if the Green Man himself does not.