DOJ Regulations provide that the AG (or DAG Rosenstein in Mueller's case, because Rosenstein is AG for purposes of appointing Mueller) can remove SC for cause, which amounts to the same reason one might remove a US Attorney from a case. Not fire them, just take them off a case, that's all it takes.
Congress certainly has the power to impeach Rosenstein, but impeaching Mueller is nonsense, becuse Mueller is not a government officer, he is a private citizen.
Somehow that doesn’t seem like a very good balance of power to me. Mueller has conflicts of interest, as does Rosenstein.
Trump has to have approval for Cabinet Officials, but the DAG can appoint a Special Counsel, who has very little check on his power. And both the DAG and SC have conflicts of interest.
Most unsatisfactory. JMHO.