It seems to be an App which opens that dialog (over a webpage...).
Try to boot into safe mode:
Press and hold the power button until the power menu appears.
Hold down 'Reboot'. (On phones which do not have the option to reboot press and hold 'Power off')
Then your phone will reboot into safe mode. That means all your apps are deactivated for this session. IF you won't get any of these weird dialogs there THEN restart your phone (to get all your apps back) and deactivate/uninstall your apps step by step.
You can also try AdAway, which has an inbuilt AdWare Scanner. (https://f-droid.org/repo/org.adaway_51.apk)