The green men have always been one of the funniest traditions. It started in Vancouver, where two people would dress up in green suits to distract and taunt the player in the penalty box. They seem to really not like the Oilers, as they turned attacks personal this time.
It then escalated further than this, as the Oilers made a formal complaint to the NHL about the green men. The Oilers were upset because the sign targeted Kane and allegations that he had been sports betting in the past, and the Oilers didn't want any distractions from the game for their players.
Even though the sign is just a funny joke, the Oilers did not take it that way at all. The NHL had to act fast because these rumours were a bad spot on the NHL when they happened, and they'd rather just everyone forget it happened. The green men will undoubtedly make a comeback cheering for the Canucks, and we'll see if the NHL is going to punish them again for their antics.