AFAIK, a powerdown will stop mover. However, it'll probably only manage to cleanly stop it after the current transfer is finished.
If (with everything else going on during a shutdown), mover takes longer than the shutdown timeout, then the system will forcibly kill the process. This will result in a partial file being on the array (in the case of useCache:yes), or a partial file being on the cache pool (useCache:prefer).
Because of how Unraid handles multiple identically named files on the array / cache, in the latter case it will appear that the file is corrupted / incomplete. (even though the source file isn't)