I have the exact same issue, and have been searching for quite some time for a fix. I'll point you to this post ( http://forum.xda-developers.com/showthread.php?t=1097434), which suggests a desktop application (MusicBrainz:  http://musicbrainz.org/) used to change your ID3 tags in your music library, then let the Google Music Manager find these "new" songs and upload them. This should (in theory) let you go onto the Music web interface and delete the giant "Unknown Album". Which should then (in theory) not ever cause your phone to try and download to offline. I haven't tried it yet, but I'm going to soon. Let us know if you get to it first.