some indie film being shot in NYC, and that Margo didn't recognize him because it COULDN'T be Greg, it just couldn't! But now that she's actually bumped into Greg and knows he's in the city, it's really unlikely she wouldn't be able to see through the old baking-soda-in-the-hair trick.
My second guess was that it would be Eric Mills (of "The Mills Gallery" fame), back from the dead after that avalanche in Tibet because, ha ha ha, no one in soap operas actually dies in avalanches, they just need some time to dig themselves out! But it's pretty unlikely that Margo wouldn't recognize her former fiance whose name she was mumbling in her sleep not too long ago. Unless his injuries from the avalanche required extensive reconstructive surgery?? Avalanches can do that, right?
Special shout out to Kibo in the comments for predicting drive me nuts.