It happens because mould loves sausage!
If it bothers you (and it should, wild mould can be dangerous!), try wiping it off with some vinegar. There is a risk that it will return, though, so I recommend you to use a controlled mould monoculture in the future. A mould culture is basically the same mould that exists on the surface of high quality salamis and cheeses, and it is not dangerous. That benign mould will prevent wild and possibly poisonous mould from forming.
There are mould cultures that can be ordered from the internet; or you can grow your own from a commercial salami.