Early on in the novel, in Chapter 1, Nick explains that he saw Gatsby as a victim of moral decay, and that moral decay clouded up Gatsby's dreams. Nick saw Gatsby as having a particularly "gorgeous" personality, meaning to Nick a personality that is complex and is highly sensitive. More importantly, Nick saw Gatsby as having "an extraordinary gift for hope, a romantic readiness" that he had never seen in anyone else and will not likely see ever again. However, unfortunately, the moral indecency of Daisy and Tom plus of anyone else Gatsby needed to associate with in order to pursue his dream of winning Daisy distorted Gatsby's "gorgeous" personality, even leading to his demise. We especially see Nick describing Gatsby as having fallen victim to moral indecency in the passage:
Gatsby turned out all right at the end; it is what preyed on Gatsby, what foul dust floated in the wake of his dreams that temporarily closed out my interest in the abortive sorrows and short-winded elations of men. (Ch. 1)
The phrase "foul dust" refers to dirty, or loathsome dust, which symbolizes moral decay. Hence, when Nick says that "foul dust floated in the wake of [Gatsby's] dreams," he is referring to the moral decay that interfered in his dreams of winning Daisy's love, showing us just how much Nick sees Gatsby as a victim.