An ad opens that takes up the entire screen for Clash of Clans. I can't figure out which app I downloaded has caused this to happen. My android version is 4.4.4 and my device is a Samsung Galaxy Alpha.
Welcome to Android Central! Unfortunately, there isn't a definitive way to figure out which app carries the adware that causes popup ads. You could try an app like Lookout's Ad-Network Scanner & Detector, but I think that usually looks for ads that pop up in your Notification Bar. You may just have to uninstall apps one by one (starting with your most recently installed) until the popups go away.