How to stop Wi-Fi from using all my mobile data? My phone all of a sudden is consuming huge amounts of mobile data ; apps usage report shows that 'Wi-fi' is causing this: from less than 500Mb per month is has consumed 2GB in a day , and it continues to do this unless  I  disable the mobile data .

What is causing this ? How can I stop it ? Can someone explain why WIFI would use my mobile data ( I thought Wi-fi would use my internet connexion)

Material used:  Samsung S5 dual SIm.
UK Three SIM card and France Orange SIM card
Probelem is with both cards and both providers. 
I've had to disable both mobile networks to stop the usage.

there are two 'Wi-Fi apps' one version 35 and a version 126.   Uninstall or Force Stop are inactive, and I cannot see who is the Publisher. 

Any suggestions ?
thanks !
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