Ah! You've not yet learned the Secret Handshake?
This is a rite of passage for anyone using Blu-ray.
Admittedly it is also a, "WHAT WERE THEY THINKING?", moment -- a perfect introduction to the world of Stupid Studio Programmer Tricks.
Anyway, there are a number of ways to escape this trap -- all of them non-intuitive. The one I use is Pop-Up Menu, which both dismisses the silly Screen Saver and also starts the movie playing again. (Technically that screen saver is implemented as a Pop-Up Menu, and so pressing that button dismisses it just like with normal Pop-Up Menus.)
Good news ahead, though! The most recent Universal Blu-ray title I've tested has a *NEW* programming style for them! Not only is this idiotic screen saver GONE, but they've ALSO replaced their incredibly annoying Menu Sound Effects with something much less ghastly.