How do I delete this app or turn it off…

For information about managing, deactivating and deleting Hide My Email addresses, see the iCloud User Guide:

Stop using, reactivate, or delete a Hide My Email address on iCloud.com - Apple Support

For more information relating specifically to iPad, see the iPad User Guide:

Create and manage Hide My Email addresses in Settings on iPad - Apple Support

It is not an app. It is a feature that you can use to keep your email address private. If you don’t want to use it, just say no to the option when offered in websites that require a log in email address and use your own email address.