It is a sponsored ad content from one of the apps. Try checking which app you downloaded latest. I had the same problem and figured out I had this app called "My camera" (some other versions are also called "Selfie Camera"). Uninstalled that and everything's back to normal now. Try looking for that "My Camera" app and uninstall it. Hopefully it'd work. ^^
Locate and click on the settings icon at the top right of the screen as illustrated in the screenshot in this thread. Then simply turn off the screensaver.