I have found a new app it calls Hide Whatsapp Status https://play.google.com/store/apps/details?id=hide.whatsappstatus&hl=es which let you chaat with people but anybody can see that your are on line or your last conexion, it is just 5MB and it doesn't desconect mobile data! wonderfull :)

You can use 3rd party apps to hide apps. Try using Hide Pictures - Hide it Pro (link).