You're lapsing into second person and rambling a little. Try something more like:
Walking by himself along the outer [inner?] edge of the crowded Manhattan sidewalk, he became the stranger no one noticed for more than a split second.
Or:
Walking along the outer edge of the Manhattan sidewalk crowded with couples, he became [is he assuming a role or should it just be "was" here?] the solitary stranger no one noticed for more than a split second.
I'm not liking "stranger" here, actually, but couldn't come up with something better. Maybe something more like "form" or something sort of impersonal. Or "solitary outcast no one wanted to notice for more than a split second." Stranger suggests noticing him long enough to acknowledge that he's unknown. And suggests weirdness, where you're going for camouflage, I think, someone who's invisible, at least in his details.
Of course, it all depends on what image and emotion you're going after, which is hard to tell out of context. But those are a couple suggestions for you to play with.
JD
