Nick is referring back to the end of chapter 1, when he sees Gatsby standing at the end of his dock staring across the bay to the green light at the end of Daisy's dock. The green light represented the distance between Jay and Daisy, but to Jay Gatsby, it represented Daisy herself. To him, Daisy was the objective. Everything he did was aimed at getting Daisy. She had become almost a dream object, or an "enchanted object," to him because he'd worked so long and so hard to get her back. Finally they were together again. Jay would no longer stand at the end of his dock gazing across the water to the green light and think of how much he wanted Daisy or what move to make next in order to achieve his dream. He had his dream; he had Daisy.
It is always important when considering questions like this one that asks you to interpret a given quote to look at what comes before and after the quote to see if this helps you pick out the significance. Note that the context of this quote comes as Gatsby and Daisy are reunited together after being separated for so long. As they go to Gatsby's house, Nick reports how ironically Gatsby mentions the gren light that marks out Daisy's house across the water. Throughout the novel, the green light stands for the hope that Gatsby has of getting Daisy back. Note what Nick tells us about this comment, directly before the quote you give:
Possibly it had occurred to him that the colossal significance of that light had now vanished forever. Compared to the great distance that had separated him from Daisy it had seemed very near to her, almost touching her. It had seemed as close as a star to the moon.
Now, however, he has Daisy, and thus it is that "his count of enchanted objects had diminished by one." The green light is therefore explicitly related to his hope of gaining Daisy, and, at this point in the novel, it appears that he has her, so the green light becomes just "a green light on a dock," rather than being imbued with all of Gatsby's longings, hopes and dreams.