What is "M resistance" inside Mosul; how important is it?

“M”, a resistance group operating inside the city of Mosul, ISIS’s de facto capital in Iraq, has been active since 2014. Recently, however, their activity has increased significantly.

In the past there has been reported resistance by the city’s people, but “M” is the only known organised force that ISIS views as a threat. The increasing power that “M” holds indicates that ISIS is losing control of the city. Indeed, the steady increase in assassinations the group are achieving as well as increasing defections from ISIS and even reports of people raising the Iraqi flag on government buildings, implies strongly that ISIS are losing control of their stronghold in Iraq.