Is this mystery app you speak of just notifying you that System UI is occurring or is it flagging it as a problem?
If the former, I'd just ignore it. System UI is integral to the Android operating system so if its popping on a regular basis that's not surprising. If the latter, uninstall that app as it's problematic and not actually helping you maintain your phone properly.
System UI used to be an option that a user could just enable so it was accessible by swiping down the Status bar. But in Google's infinite wisdom it eliminated the toggle to enable/disable its viewability, the gist being Google thought it best to limit options for the users to alter user interface options to the Settings menus. Not unlike the Developers Options menus, the System UI menu is always there, it's just a matter of toggling its visibility to the user one way or the other. You can still make it visible, it's just not as seamlessly easy as it used to be. Oddly Google has left the Developers Options menu to be a user accessible option if they care to.