The "green light" in The Great Gatsby , in terms of the one that appeared in the very first chapter("...I glanced seaward-and distinguished nothing except a single green light..when I looked once more for Gatsby he had vanished...(pg.21)"), represents the greed and envy for the American Dream. In other words, the corruption of it. In the beginning Gatsby is staring out across the bay and there is a green light that appears when he vanishes the next second. I feel that this could represent the greed he felt for Daisy and the envy he had of Tom. It also could represent foreshadowing of his plan to get Daisy back throughout the book.
The "Valley of Ashes" in The Great Gatsby could symbolize the motif of the American Dream, more specifically the what the Old American Dream used to be. On page 23, the valley of ashes is first introduced("...a fantastic farm where ashes grows like wheat into ridges and hills..."There is little to nothing left from location and all that is really notable are the dead buildings, Wilson's Garage, and the billboard of T.J. Eckleberg. This symbolizes the life left behind when they moved on to the fast-paced lifestyle. It's so fast that the people never stopped to look back and left all of that behind, leaving a drastic change in the Dreams.