I should have known a FReeper would have the answer at the ready!
So he goes before the GJ and take the fifth.
According to a link in the article the judge says Mueller’s appointmentment is legitimate because he is supervised by a constitutionaly appointed officer. Levin made the case Mueller had the powers of constitutionally appointed official without being confirmed by the senate. Mueller is running his own office with broad powers. It is not like he is just an aid to Rosenstein.