Well, it's not harmful to the pant, as it should only grow on top of the soil if you keep it too damp or moist.
However, it is a type of mold.. So I'd do as leafy suggest.. If you just opened the bag then go get a new one.. One not advertised for being "moist" in the bag.
Also, scrape top 1cm of soil in your pot and replace it with something new. It'll get rid of it in the pot. Just water less often and it shouldn't show up again..