Considering both .musx and .mus file are from Finale music notation software, your best choice for musx to mus conversion is to to use the original program, to be more specific Finale 2014 or later.
It looks like that music notation export from newer format (.musx) to older one (.mus) is doable, because Finale 2014 introduced backward compatibility and since this versions users can export musx to mus using the File ► Export ► Finale 2012 function.