I made some Swiss meringue buttercream about a month ago and I put it in an airtight container in the back part of my fridge…  is it still good? 

 I took it out and tasted it and it's great!  With that much butter can it go bad?  I want to use it for cake balls...

It can go bad but it's the sugar that helps preserve it.  If it tastes okay to you it's fine, but better to store it in the freezer long term than the fridge.