I don't see any evidence of an oil leak in your picture, if the cam cover gasket was leaking its usually from the side of the cover and drips onto the exhaust heat shield where it burns and produces oil smoke in the engine bay. The other location for leaks is the oil filter housing to engine block joint where the gasket turns hard after many heat cycles and old age. The cam cover gasket suffers the same, the centre plug well gaskets also fail and allow oil into the plug wells causing problems with igntion as the plugs get covered in oil.
Z4's seem to attract dust into the engine bay, I think thats what your seeing, no harm in jet washing the engine bay but cover the Alternator with plastic to keep it dry when you do it.