Not sure about an active culture since you can't really mix anything in but for future reference what I do to set up new cultures is this. I combine 3 parts water with 1 part white vinegar and then use that to mix my media and no mold. Although I also use cinnamon in my media mix and a few times I forgot to add the vinegar and no mold in there either. Supposedly the cinnamon helps keep it down as well.
Other than removing it like Mysty says I'm not sure what you can do in an active culture. You will never get it all though so it will probably just come back. Probably best to just set up a new one.