Each device that is associated with a Glacier account generates its own "device fingerprint". In some situations a message could be delivered as "Message cannot be decrypted". This can happen if device keys are out of sync, the receiving party has recently logged into a new device or regenerated their keys from the app settings page.
In most cases sending a message back to the other user will re-sync your connection. If that fails try these troubleshooting steps:
After logging into Glacier on your Android 8.0+ the app will show a message in the notification tray "Glacier is active". This message is enforced by the Google Play Store.
To hide this notification: