According to the name, it's not just the folder name - it's an alternate data stream, associated with the folder.
But it certainly is weird... don't know why avast! should report the stream, but simultaneously find it invalid (the displayed name doesn't contain any special characters, so if it's complete, it should be OK). Besides, there's no reason any ADS should be present, of course...
I might guess a filesystem problem, but if chkdsk doesn't report anything...