On my N20U you can do this by enabling developer options, then enable "Force allow apps on external" from developer options and then you should be able to move an app by either long pressing on the app icon and selecting App Info->Storage or going into settings->Apps->Whatsapp->Storage.
To enable developer options click on build number in settings->about phone->software version seven times.
Up to you of course and entirely at your own risk but I've done this quite a few times with apps, just take care not to change any other settings in developer options that you don't know about. The setting you want is about three quarters of the way down. Apps on the sd card will load a little slower depending on the sd card you have