Heavy green gas permeates a Minnesota neighborhood, crawling along the ground as a suspected carcinogenic agent.

[ProfessorGAC]

7. It's A Weird Shade Of Green For Chlorine

Besides, on exposure to moisture in air it goes to an equilibrium between Cl2 & HCl (favoring the right side) & HCl is white to clear.
And that green looks to dark for chlorine. Chlorine is closer to what I would call chartreuse. Way more yellow in in than sheer green.
This almost looks like some sort of dye to make a leak of a noxious compounds visible. Sort of like adding methyl mercaptan to natural gas to give a detectable odor.
I can't immediately conjure any thoughts of a gas that would present green.