Can mold spread to unopened butter?
Butter is mainly fat - no nitrogen. (It is about 0.8% protein by weight.)  So it is not a very nutritive environment. But its principle protection is that it is “dry”. Although it is about 16% water, that water is emulsified and therefore protected from contamination.  The original wax paper wrapping, and the surrounding container, as well as the temperature of the fridge, would make it very difficult for microbial contamination or growth to occur.