The term comes from the NBA 2K series. In those games, the quality of a basketball shot is measured with a colored meter. Green signifies the most effective shot, but it is also the most difficult to hit. The FN means "fuck n--ga"
‘Dot me, Dot me!!… ‘Green fn’ the corner sitter shouted after lacing a wide open 3 in the corner (the corner sitter been waiting for the pass since 1763)