How to stop Wi-Fi from using all my mobile data?
As suggested by Dan Hulme, this is a malware app that called itself 'Wi-Fi'.  After his comments, I read the below post by Srihari Karanth that helped me to identify that ''Wi-Fi'' was in fact a Mobile tracker app. 

Srihari's post

''To know if a spy app is installed first check your Device Administrator list to see if there are any apps which you have not downloaded yourself. If you find any apps, no matter the name, no matter the icon of the app, just disable the privilege and uninstall it from application list as valid system applications will not be listed in there. Device Administrator list can be found under Settings -> Security or Settings -> Application.''

thank you