Very close, but you should somehow represent the fact that the entire area around the close button - anything that isn't a black pixel - is a big "open a pop-up window to the advertiser's site" button.
What's really fun is how YouTube has an X to remove the videogames (!?) it has. And tapping on it doesn't do anything other than change the buttons color for a split second.
And even if you manage to hit it you are thrown in a slightly different looking add of the same thing, with a different x button. And guess what? Theres a third one right behind that x