If it was a courts-martial, then it was a military officer as military judge. the Army has many military judges who work and are assigned directed from the Army judiciary in Falls Church, VA.
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Can Mattis fire the judge?
Any outrage from McCain?
No. The judge could be challenged for cause by either the Trial Counsel (the army prosecutor) or the DC (defense counsel). I only saw it once in my career and the judge denied the challenge. It goes in the record of trial (which is done by a military court reporter and they type out a verbatim record of everything said in court and that sometimes massive record goes to the appeals courts. I have seen murder cases where the record of trial was several volumes and each might be 8 to 12 inches thick!!!!!) of course and can be part of the review by the Military Court of Review which is the first line of appeals. You can appeal further to COMA (court of military appeals, the final stop, UNLESS you have something that the US Supreme Court might take, by that is way more than rare. I can only remember one case going to the Supremes, that I read about. It was not our case.) That is the way it was when I retired in 1992, I assume it is probably still the same some what.