To remove WhatsApp from your hidden apps, go to the built-in settings to hide apps on your phone. Open the Settings app to find the option, then you can unhide the app from there. If you can't find such option, check your launcher's settings or Secure Folder.
Now, as the app is stuck on the uninstall process, go to Settings > Apps > WhatsApp > Storage > Clear Data and choose Uninstall again. If it is still stuck, reboot your phone.