It just oxidised or something like that In theory Its not safe, because it has bacteria from all the processing plants on it and meat from thousands of different cows and etc, which might make it bad if it overpopulates In theory In praxis, Ive eaten lot of grey ground beef and would do it again if I happened to have ground meat, which isn't best. It oxidies or rots faster than regular meat for some reason, so it turns grey and such, but its probably practically safe to eat