no info on ur car, a lot of people here are Altima guys, and most are not prone to guessing if they don't have any first hand exp.
looks to be a vent release for gas fumes in tank. sure the reason it looks like something hooks up there is because it most likely has a service tool that checks the pressure of the valve under certain conditions. or maybe a emissions control device for other countries... clean because gas is a really good stripper/cleaner...
attached pic of mine, looks close to the same other than white piece, which might just be different year or engine model.
been while since I've looked at the EVAP system, believe it is operated of the vacuum off the engine. refer to FSM to see where all the areas are to check for leaks.
here is link to same code http://www.nissanclub.com/forums/b15...uel-issue.html