In The Great Gatsby, what does the "foul dust" that "floated in the wake of [Gatsby's] dreams" mean?

This line refers to the ending of The Great Gatsby and how deeply it affected Nick Carraway. In chapter 9 Nick explains why he moved away after the funeral by saying, "After Gatsby’s death the East was haunted for me like that, distorted beyond my eyes’ power of correction." At the end of the book Nick is no longer able to separate Long Island from his memories of the distasteful people living there. 

No one, except for Nick and Gatsby's father, came to the funeral. All the roaring, exciting parties that Gatsby had thrown, and not a single one of those people who had shown up and indulged in his overflowing hospitality had come to his funeral. Not even Gatsby's business associates and supposedly close friends made an appearance. Only one man showed up to the cemetery, and even he was appalled that no one else had come. 

The fact that no one seemed to care about Gatsby the second he died clearly upsets Nick immensely. Nick even mentions that living in his house, so close to Gatsby's abandoned mansion, disturbs him. 

"I spent my Saturday nights in New York because those gleaming, dazzling parties of his were with me so vividly that I could still hear the music and the laughter, faint and incessant, from his garden, and the cars going up and down his drive."

All these things contribute to the "foul dust," no doubt akin to an eternal bad taste in one's mouth, that Nick speaks of in the beginning of the book.

Perhaps the thing that drives him away from the East the most is the Buchanans. Daisy never once called or wrote, or gave any indication that she even knew Gatsby or cared in the slightest. One day in October some time after the funeral, Nick mentions meeting Tom and he acts extremely cold to him. Tom defends his behavior and denies any culpability for what happened to Gatsby.  After that meeting Nick's opinion of the Buchanans is cemented. 

"They were careless people, Tom and Daisy — they smashed up things and creatures and then retreated back into their money or their vast carelessness, or whatever it was that kept them together, and let other people clean up the mess they had made...."

At the end of the book, Nick is thoroughly disgusted with everyone around him and the magic of East and West Egg had died along with Gatsby. Those lines at the beginning of the book are about Nick's distaste for everything that contributed to or failed to care about Gatsby's death.