Zelena karta is the green insurance card you may be asked for at the border.
The image is as short as the word: there are Lego bricks lying on the card, and they are soaking wet. Lego and wet (nass in German) give you zelena, and karta takes care of itself.
Some words do not need a long story – if the sound is already close to a familiar one, a single image is enough.