Is it OK to cover a cold ganache cake with MMF or do I wait for it to come to room temp?

Room temp.

Edit: are your fillings perishable? If so, you have no choice but to work with and maintain a cold cake.

Vgcea thank you for replying. So I wait and let it come to room temp then apply the fondant. Also do I let it sit out or put the cake covered in fondant in the fridge?

Why is it in the fridge, and for how long?

I put mine briefly in the fridge to help the ganache set, but it doesn't get cold all the way through. If you have lots of time, you don't need to put it in the fridge at all. Just let it sit out to set up.

Unless you have soemthing like custard or whatever as a filling - then it needs fridging.

Mcaulir I had it in the fridge and took it out. Now it is kind of tacky to the touch and the ganache is not hard. Do I let it sit overnight on the counter and cover it with fondant in the morning? Will it firm and set over night?

One common mistake people make when working with chocolate is this whole refrigeration thing. Yes if you place your freshly ganached cake or whatever form your ganache is in the fridge, it hardens because you've shocked the chocolate into crystallization. The problem is the crystalline structure formed this way is crappier than those form V crystals that properly handled chocolate forms when allowed to set properly. So when you bring it out of the fridge, those unstable crystals become just that unstable, and you end up with ganache/chocolate that does not set, and stays soft/melty at room temp.

If you must refrigerate, a few minutes should be enough, then just let the ganache set at room temp.