If you pull down the notification bar and slide the notification message to the right you can access the notification settings via the cog symbol.
Alternatively this notification comes from the Huawei 'Files' app, find it in apps and notifications and kill that app!.
According to Huawei support, starting with EMUI 5.0 (Android 7.0 Nougat), moving apps to SD Card is no longer supported. Period. Really bad decision if you ask me.
Posed the question again on https://www.facebook.com/usahonor in the 'Flagship' question box and got the answer was that it is as of EMUI 4.1 that apps on SD card are no longer ...
You can try the following method proposed by pruther69 here and this method should allow app data to be saved to the SD Card:
On your Huawei device navigate to Settings ➡ Apps ➡ Advanced (bottom of screen) ➡ App Permissions ➡ Storage
Select respective application from list (this should then allow data for that app to be saved to the SD Card)
To transfer ...
Since you have a the stock ROM you should do this:
From TWRP press Advanced
At Fastboot mode now
Connect to a Windows Computer. Assuming the computer has Minimal adb and fastboot and you already have the Universal USB drivers installed on it
On the computer extract the ROM to the folder, C:\ProgramFiles\Minimal ADB And Fastboot
Back to the ...