What's green and falls up?
Seaweed is green and can be seen as falling up, because when it falls off the plant it's attached to, it rises to the surface.
Another answer is Earth, because people tend to refer to nature as green, and the gravity of planet Earth also pushes up to meet people, buildings, trees, etc. The gravity of our bodies and other objects above ground, as well as the gravity of the Sun, pulls Earth upwards to some extent.
Cruder answers are "vomit" and "phlegm".
Some suggest that this is just a joke question. One keeps repeating the question until their victim gives up and asks "What?" to which you reply "I don't know, that's why I'm asking you!"