Nick is sitting out behind his house at night, having just returned from his dinner with Daisy, Tom, and Jordan. He's left his car and is looking out over the water, apparently processing the night's events. He sees a figure near the Gatsby mansion and assumes from its way of walking and standing that it is Gatsby himself.
Given that Jordan knows Gatsby, Nick decides that's enough of an introduction and is ready to call out to him. But, as Nick puts it:
he gave a sudden intimation that he was content to be alone—he stretched out his arms toward the dark water in a curious way.
What also stops Nick is that Gatsby appears to be trembling: this suggests that he is emotionally moved. Nick realizes this is a private moment for his neighbor, and he has the sensitivity not to interfere.
There's a ghostly, mysterious quality to Gatsby on his first sighting. We don't know why he is reaching out toward the green light or what exactly makes him tremble. And then, all of a sudden, he seems to disappear:
When I looked once more for Gatsby he had vanished, and I was alone again in the unquiet darkness.