It RNG. You have to go talk with him. There a chance that you will see him take a bath at waterfall (you don't need to check to waterfall, it just an event). During dialogues, there an eye icon appear at top-left of the screen. You must click on that to see CG.
You get it when your done with the quest, When Nauxus in the waterfall and preparing to go you'll get prompt to look at Nauxus Bathing in the waterfall
