What did Nam see in the greenhouse?

The polar bear (or some other form of ice and snow dwelling animal). The reason he withheld the fact that he saw the polar bear was because the idea that the snow could ever be melting was already such an impossible notion to the dwellers of the train who had spent years in the complete belief that the climate change was permanent, gasp that telling Curtis he saw a polar bear (or any sign of life outside the train) would be meaningless. Besides that, he's completely aware that Curtis sees him as an addict that's too hopped up on drugs to know up from down, so his findings would essentially be falling on def ears. It wouldn't have benefitted anyone under these circumstances. Besides, it made for a strong ending :)

This is very late in coming, but I've only just become familiar with this film. Nam says he knew the weather was changing because of the plane crash remnants he looks at every year at the Yakaterina bridge. His flashback of seeing the snowflake is different from the one where he looks out the greenhouse window. There is a point when we're looking out a window (I can't tell you exactly where, but it's getting toward the end) where we see a small area of blue water. IMO, he either saw the polar bear or some other animal, or water.

I believe, and I could be wrong, that he recognized the type of flake that came through the window. He mentioned something about knowing it was getting warmer because of the crystal structure of the flakes. That's what he saw in the greenhouse.

When they a having sushi he is looking out the window and sees the ships have very little snow and ice on them. As well there is tower where you with no snow on it and you can clearly see a blue stripe. Also far in the distance you can see a red and white light house clearly has no snow on it. This confirms his theory that the snow is melting.

At first I thought a polar bear, but that would be too cliché.

Then I thought he saw ground, because when Yona asked why the soil was moving, he was in the middle of the sentence "it lives under the -" when he moved to the window saying "what's that" and the scene cuts.

I think my next guess was water, because when the scene cuts, the next thing camera pans towards was the aquarium - water! But that would be too meaningful that he wouldn't resist saying it out in the end.

So my best guess : icicles! He saw icicles! Because the next time they showed the outside was when they ate sushi. Nam just numbly looked out the window. There was a shipwreck and all of the landscape was covered in icicles. And icicles are the symbol of melting ice. That's why it was so meaningful, yet so meaningless.