The message "Application: decode items from other devices" appears in the Actions sub-tab of the Dashboard when another of your devices is in the midst of uploading data. When a device is uploading, it notifies the other devices on the account that there is new metadata to be downloaded from the server. It does this so that each device's Manage tab is updated and you can browse or download anything from the account. Short appearances of this message are normal and nothing to worry about.
If the "decode items from other devices" message remains for a long period of time, you should check your other devices to see if one has a stuck upload or is trapped in a never-ending loop of attempting to upload data that is constantly changing. If you find a device with such a problem, clear its upload queue and de-select from backup all data that are constantly changing. Once other devices' uploads are complete, the "decode items" message should clear.