Here are some instructions on how to force close the app

On your iPhone X, from the Home screen, swipe up from the bottom of the screen and pause slightly in the middle of the screen. Swipe right or left to find the app that you want to close.  

When your recently used apps appear, the apps aren’t open, but they're in standby mode to help you navigate and multitask. You should force an app to close only if it’s unresponsive.