The Green Fish is a fish Pocket God (Facebook).
When the player drops a Bomb Fruit into the water and it explodes, four fish float to the top dead, one of them being a green fish (along with a Blue Fish, Orange Fish, and Angler Fis). The dead fish then sinks to the bottom of the ocean from where they had come from. It is yet to be seen if the creators embrace having the green fish to interact with other than after it is exploded.
This is also one of the fish that a pygmy can now catch.