Can I move an app back if my external drive isn't connected?
Short answer: No. The app is on the drive, so reconnect it to move the app back. If the drive is lost, reinstall the app — its settings stayed on your Mac.
No — the app itself lives on the drive, so it must be connected to copy it back. While the drive is unplugged, the Moved tab tells you to reconnect it.
If the drive is lost or has failed, the app can't be recovered from it; reinstall the app from its developer or the App Store instead. Your app's settings and data stayed on your Mac with a Move, so a fresh install usually picks them up.