The Nexus 5 comes with a 1.2A (1200 mA) charger. If the charger in question is a 9W then it is rated at 1.8A (1800 mA). The charger in question is capable of putting out 1.8A that does not mean it will send 1.8A to the device being charged. The device being charged only pulls so much current from the charger. The amount of current the phone will pull is 1.2A (actually less) or less other wise it would over load the charger. When using a charger to replace the factory charger always make sure the replacement charger is rated the same as the factory charger or Higher.
So to answer your question. Yes, you could use this charger.