[the smiling weirwood]

at one point (I can't remember where) it says that when the Children were at Moat Cailin trying to bring down the Hammer of Waters on the Andals they became close to the crannogmen(possible intermarriage?) and taught them many of their secrets, such as running on leaves, breathing mud, and becoming invisible.(which Meera and other crannogmen DO possess) I'm guessing the green dreams are just a rarer manifestation of this connection to the Children.