The "green stuff" that is on magnesium parts is zinc chromate primer. It is put on to protect the magnesium from corroding.
See below:
Zinc chromate is simply a corrosion resistant pigment that is added to certain coatings. In the case of a wash primer, the phosphoric acid actually reacts with the metal, whereas in the case of zinc chromate this is not the case. In dry, low humidity weather the zinc chromate does not do anything other than remain in the coating. On the other hand, when the humidity in the air increases, or on a rainy day moisture for the air penetrates the primer coating and slightly dissolves the zinc chromate. The dissolved zinc chromate solution now does react with the underlying metal surface and forms a passive layer (like a blanket|) that prevents corrosion. As soon as the weather becomes dry again the zinc chromate no longer pays a role ... at least until the next time it rains, when the process is repeated. If there are frequent cycles of high and low humidly the zinc chromate will eventually be depleted and soon after you might start to see corrosion of the metal..
Zinc chromate primers are most often used in aircraft and military applications.
Aircraft Spruce in the USA carries them.
http://www.aircraftspruce.com/catalo...mateprimer.php I would expect that similar companies in the UK would as well.
Brian