Why does my buttercream become runny overnight? I made this very simple 'meringue' butter cream. 
40 grams of egg white, mixed with 80 grams of icing sugar over an heated bowl until the nice peaks are there, like you want with meringue. Add a little bit of salt and then mix 120 grams of butter in. 
I used a bit of vanilla extract and icing coloring. 

It all went fine and I had the consistency I wanted and could work with it very well. 
However I decided to do most of the cupcakes the next day. I left the buttercream in the piping bag in the refrigerator overnight. 
When i wanted to use it the next day it was 'runny' when it was back at room temperature. It was turned in almost some kind of thick liquid. Mixing it again did not help. I red that storing in the refrigerator should not be a problem, however this was for 'normal' butter cream.  

Where did it go wrong, and how can I prevent this next time? 
Did I store it wrong? Or is the consistency wrong? Or should I handle this 'meringue butter cream' different from normal butter cream?

Thanks in advanced!
Meringue is a protein-based foam. The proteins form a semi-stable mesh with air bubbles trapped inside. The reason they are semi-stable is that they unravel a bit and hook into each other, Velcro-style. When you introduce fat, it lubricates and smooths the unraveled proteins, so they lose their hooking ability. If the fat is introduced before the eggwhites are beaten, there is a fair chance that you won't get a foam at all. 

It is possible to mix the prepared foam with fat without destroying it, although it will lose some volume. This is usually done when the bulk of the mix is not the meringue, but some kind of creamy filling which gets lightened by the air added through the meringue. These fillings are usually set through baking (such as a cheesecake filling), or gelling agents (there are cremes which combine egg whites foam, fat-containing dairy products, and gelatine). As you did not set the icing in any way, the fat from the butter slowly worked itself into the formed hooks of the proteins, destroying the mesh and returning the eggwhites to their original form (a suspension of proteins in water). Eventually, if you overbeat the meringue a bit, you also got some weeping caused by the proteins slowly getting tight enough to expell some water from the structure, but if present at all, this is probably a secondary effect in this case. 

Bottom line: the mix you made is inherently unstable. I wouldn't use it as an icing unless the food is eaten quickly. You should get a different recipe. If pure buttercream is too rich for your taste, you can try versions which add yolks or whole eggs (not foamed eggwhites). The lecithine in the yolks emulsifies the rest of the egg with the butter, so you don't get separation problems. Another option is to forego buttercream completely and use whipped cream, it still gives you a rich mouthfeel, but has much less fat per weight and whips to a higher volume.