A/C components have a green o-ring where lines, condensers and high and low pressure switches connect to the system. They are a special o-ring designed to handle the temps and high pressures of the system. They are considered single use and should be replaced any time they have been in a pressurized system. In a bind I have reused them, but I would not in my own vehicle and that one is surely damaged with that leak, so it needs replaced. But more than likely a new switch would come with one.
Also, the oil isn't actually green, it comes out that way but quickly turns back to a yellow/amber colored oil. It could be a dye or some reaction to air, I am not sure.