Green FN stands for a far more explicit phrase, "green fuckin' nigga" that's used to refer to someone who is particularly suave or
cool. In its original use case, the term was fairly specific to the world of
basketball. That's because the term originated from NBA 2K, a series of video games in which gamers get the chance to replicate an NBA season and play with or against the stars of the modern NBA.
In NBA 2K, a green shot is one where you nail the timing and release so that it has a
high probability of going in. Green shots are considered the goal for many who play the
game, a "Green FN" is a person who brings that kind of swagger to their life off the court, in addition to their
game.
Essentially, then, Green FN is a term that started in the world of gaming, and was eventually applied to
real basketball and then to other circumstances outside of basketball.