If you must have a tab, it should have a big dismiss button for the 90% of visitors who are not interested. Why is there no option to close it?

Another option would be not to have a tab at all (a hovering tab is very annoying as it hides content and distracts from the site for a function which is not often used) - instead let customers include an unobtrusive help button in their actual page content wherever they wish which calls your js in order to display your overlay when clicked. You don't need an obnoxious tab.