The Muslims showing solidarity with Catholics sends out a message to the terrorists that decent people of the world will not be turned against each other by these heinous attacks carried out in the name of an ugly and ill-informed interpretation of the Islamic faith. If the terrorists end up bringing people together instead of setting them against each other there is no incentive for them to continue these attacks.
Just because peaceful Muslims showed support to show that their religion is not one that condones violence does not mean the war on terror is slowing down. There are extremists that don't seem to be letting up. The most this act is doing is showing that these acts of terror are not in the name of Islam. These are radicals using Islam as an excuse for their actions even though most Muslims don't believe or act this way at all.
The War on Terror doesn't "turn" because of a gesture like this. It takes years and even decades of this type of activity from all sides to make an actual dent in the impact of terrorist groups around the world. It's a start, but it's not the first and it will definitely need to not be the last.
No, Muslims attending Mass is not a turning point in the war on terror as the war on terror has never really been about religion. While many Christians see Islam as the enemy we have been fighting thanks to propaganda and fear-mongering, that's not the case. Instances of solidarity like this should, however, be publicized as a way to change the prejudice against Islam the war on terror has caused.