Who is that "ashen fantastic figure gliding toward Gatsby through the amorphous trees?"

The entire quote:

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He must have felt that he had lost the old warm
world, paid a high price for living too long with a single dream. He must have looked up
at an unfamiliar sky through frightening leaves and shivered as he found what a
grotesque thing a rose is and how raw the sunlight was upon the scarcely created grass.
A new world, material without being real, where poor ghosts, breathing dreams like air,
drifted fortuitously about...like that ashen, fantastic figure gliding toward him
through the amorphous
trees."

This comes from
chapter 8. Myrtle Wilson, Tom Buchanan's mistress, has been run over by a car. It was
Gatsby's car, but Daisy was driving it. Daisy and Gatsby and Nick, Jordan and Tom had
been driving back to Long Island after being in NYC where Gatsby forced Daisy to tell
Tom she never loved him, and she really could not say that she never
loved him. When they passed by Myrtle and George Wilson's garage, Myrtle, who
had been locked upstairs by George, her husband, because George had learned Myrtle was
having an affair, saw Gatsby's car from the window. She thought it was Tom and ran down
to talk to him, ran in front of the car, and Daisy ran her over. George does not know
WHO killed his wife, but Tom, who figures out what happened, comes upon the accident
because he is driving behind Gatsby and Daisy. He has been trying to sell George a car
and knows George, so he whispers something to him. Later, we find out that he no doubt
told George that the car that hit Myrtle was Gatsby's car. George assumes that Gatsby is
Myrtle's lover, not Tom.

So, the "ashen figure" is George.
He has gone to Long Island to find the car. He figures out that it belongs to Gatsby.
Gatsby, who has never used his pool all summer, decides to have one last swim before he
lets his servants drain it, and Geroge is hiding in the trees, waiting for him. He rises
up from the trees and kills Gatsby.