The good news is that this seems to have happened.
120 Muslim scholars, including the Grand Mufti of Egypt, the dean of Sharia and Law at al-Azhar University, director of the Fiqh Council in the US, and the Grand Mufti of Jerusalem published an open letter to IS leader Abu Bakr al-Baghdadi decrying his state’s un-Islamic behavior in three languages: Arabic, English and German.
The letter is here.
I find it interesting that some of their arguments against Al Baghdadi seem also to apply to Hamas and many other terror movements that are considered praiseworthy by avergae Muslims, such as a the prohibition of killing innocents, that Jihad is meant to be defensive war only, the prohibition to mistreat "people of the Scripture" (Muslims and Jews,) the prohibition to deny women and children their rights, and the prohibition against torture.
That little bit of hypocrisy might weigh against people believing these scholars. But it is definitely good news that this happened, and the response will be worth following.
Notably, the scholars only call on Al Baghdadi to repent.
UPDATE: Robert Spencer shows that the "moderate" clerics are not exactly moderate by normal standards. Even so, I like to see jihadis put on the defensive from their own people; the problem of Islamism altogether is a much bigger issue and one that is not at all addressed by this letter. (h/t Henrik)