Cracking the cream cheese frosting!
Cream cheese icing is something we are all familiar with and likely first tackled when we were at school, probably to wildly varying results! Although it's easy enough to achieve, there are a couple of rules you may want to be aware of!
Firstly, before adding the cream cheese to your butter mix, make sure you drain away any water. Excess water is the bane of any emulsion; as a fusion of two ingredients with high amounts of fat, the butter-cream cheese combo won't be able to absorb any significant amount of water, so any excess will pool in your bowl, leaving you with soggy, weeping icing!
The second tip is to beat the cream cheese into the mixture until just combined, rather than whipping the life out of it! We already incorporated plenty of air into our frosting when we initially whipped up the butter and overbeating the cream cheese will cause the icing to become runny. Should this happen to you, place the icing in the fridge to chill for as long as you can and check on the consistency after that; hopefully the butter in the icing will have firmed up enough to make your concoction pipeable!
If it's still a bit loose for your liking, then you may be able to thicken it up by adding more icing sugar, but to be honest it will make the icing a lot more sweet, likely cloyingy so, so it may be in your best interest to consider restarting at that point!
It's worth noting that overbeating of the icing most often occurs if you are struggling to get the cream cheese to blend smoothly into the butter, leaving you desperately trying to beat the lumps of cheese into order. If that tends to be a problem you have, try bringing the cream cheese up to room temperature before you add it. If the butter and cheese are at the same temperature, they will combine much more smoothly and may resolve your frosting fears!