I think I read somewhere that the green stuff is high in silicates which is abraisive to aluminum automobile parts. Thus, it is possible that the green stuff actually caused the water pump to fail. It would be interesting to look at the old water pump to see if this is the case. Crud from this corrosion process may have sealed the leak and when you changed the coolant back to the MB stuff, it dissolved it. Will the green stuff make your water pump die immediately upon contact. No, but it probably will accelerate the wear of the water pump.