Video clips on the mobile app don't have a close button. I've tapped and swiped everywhere but the only way to close them seems to be to turn off the mobile and turn it back on.
What mobile device are you using? The iOS app should have a back button in the upper left corner of the screen while the Android app uses the back button below the screen. In either case you should be able to exit the app by pressing the home button.
No back button is visible. Clicking the corners of the screen does nothing. The app returns to the same point after clicking home. The only way to close the clip is to turn the mobile off and restart.