If it is an app you downloaded to the phone first and then moved to the storage, it will delete both if you try to delete one. Also note that once the card is converted over it is then "Internal" storage, not "External". That is if you did it correctly. I will include how as well.
The smartest way is to set up the card as "Internal" storage and then download any apps after that, directly to it. Then there is nothing on the phone itself.
NOTE: That phone, as far as I know, doesn't allow for moving the apps to the card, but it could be an older version that wasn't restricted.
How to turn external SD card into internal storage on "Marshmallow".
NOTE: Please be sure you start with a blank SD card. It needs to be blank because when the card is converted to "Internal" storage, it will be formatted (wiped) and encoded (making the cards data only readable by that device).
1. Go to device “Settings”, then select “Storage”.
2. Select your "SD Card", then tap the “three-dot menu“ (top-right), now select “Settings” from in there.
3. Now select “Format as internal”, and then “Erase & Format”.
4. Your SD Card will now be formatted as internal storage.
5. Reboot your phone.
NOTE: If you don't reboot the phone, many things may not work correctly, so make sure you do.