How to fix leaking fitting under sink?

Tywoodpk

I discovered the fitting under my sink is leaking.  It’s a drip from the brass fitting where the arrow is pointing in the picture.   
it only leaks when running the tap. 
Is this something I can replace myself or somehow stop the leak or do I need to get a plumber? 
I’m not sure what the grey thing is to the right, but that goes down to the dishwasher, I think.  The copper pipe sticking up goes nowhere,  so I’m not sure why it’s there or what it does. 
thanks in advance for any help or suggestions. 

JDE

Tywoodpk

This whole set up is just to get water to your dishwasher. The pipe going up is a antihammer device. It could be put anywhere. The grey piece would be the pressure regulator for the dishwasher. In order I would try 1. Tightening up nut where dripping. 2. Switch off water with tap immediately next to it, check rubber washer inside and replace it (plumbing section Bunnings) and use some plumbing tape on thread. Or you could  try just unscrewing it putting some plumbers tape on thread and screwing back up. Or remove whole centre section and screw grey part directly to tap, this will remove the hammer arestor and may cause hammering but arestor can be put elsewhere. Most houses do not have an arestor unless water pressure is really high.

JDE