Oil Leak on 2007 Mercedes-Benz GL450

There is an oil leak at the end of the valve cover facing the rear of the car. There appears to be a black plastic round cover about 1 1/2" in diameter where the oil leaks around. Anybody know what that is? Is it just a plug of some kind? Part of the valve cover gasket?

Asked by for the 2007 Mercedes-Benz GL450
I had a similar leak and it was the cam plugs. I had them replaced and no more leaks.