That's not possible. At least not with highlighting. You can use a different technique that allows you to select a value in a for example listbox and then highlights the corresponding expression and can grey out the other expressions.
See this blog post for an example: Line Chart tricks: Displaying what's important.
The blog post example uses dimension values, but you can apply that kind of formula to expressions background color settings too. If you need more help then please post a small qlikview document that illustrates the problem.