Adult SciFi. Underground people w/ green skin want to stop humans from destroying mountains by creating hybrids

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Adult SciFi. Underground people w/ green skin want to stop humans from destroying mountains by creating hybrids

1celinalin
gen. 22, 2:16pm

My friend from Brazil read this book around 1994.
She thought the book was by Jules Verne, but she hasnt found any of his works that resemble this plot. But maybe the author of the book Im looking for writes in a similar style?
The cover is half white, on the bottom half there is a green field and in the bottom middle a boy with green skin. (obv there could be other covers since this could have been a brazilian edition).

PLOT
The book starts with a boy going to school and always having felt different and like an outsider. Turns out he can see these green creatures that no one else can.
Later on its explained that humans are destroying mountains (i think) and that theres an underground civilization of people that want to stop them. Im not sure if they are supposed to be humans or more alien like. They have green skin because they have an artificial sun. They are more intelligent than humans, and have a more community-like system. They know that war isnt a solution.
Their plan is to mate humans with their species to create people that will understand both sides. My friend mentioned something about the book describing their mating system really well. They would sneak into couples rooms after they had sex to take out the eggs and sperm.
They implanted a lot of embyos into human females but only 1 survived. This is the boy from the beginning that feels like an outcast. He grows up in the human world, looking like human too.
This green skinned species end up adopting him. He learns the way they live. His skin also turns green from living there for that long. In their civilzation it isnt only two parents taking care of a child. A group of individuals take care of one kid.
The boy sees what humans are doing to the earth/mountains and he wants to help to stop them.

Any suggestions are helpful since we might have gotten some things mixed up. Thank you! :)