But if there is milk in the BC does that make a difference? Would the milk spoil? I sometimes add milk to my BC to thin it.
The milk in the frosting will not make any difference. Once the milk is mixed in the sugar molecules bond with the water molecules in such a way that there are no "free water molecules" that was encourage bacterial growth.
While not the same, it's similar in concept to freezing food. The water is in a state that cannot be readily utilized by bacteria.
This is a theory known as water activity control. It's basically used to determine the the shelf life of various food items (along with other variables). It's measured on a scale of 0 to 1.0 (where 0 is really really dry and 1.0 being pure water).
I would not worry too much about the icing on the cake spoiling. The cake will spoil faster than the icing.