I see that a lot of you use ganache as a filler in your fondant covered cakes, is this safe to leave out because it is made with heavy cream, or is there a different recipe.

Also, I found a recipe for mocha buttercream which has milk, is this safe to use with MMF?

I am still learning and trying to experiment with the different flavors. I wan to make a choc. espresso cake with mocha BC covered with MMF.

The short answer is "yes", to is it safe to leave out. The amount of sugar used acts as a retardant. But, I would still recommend the cake being in the fridge until close to serving time. If the cake is out for a couple of hours or, it'll be just fine.