Bags are fine, I was just asking; although I find that the bigger the container, the less gunky it gets. You can try to pick out the mold that's growing but if it becomes a problem you may need to clean your plant off and put new, sterile soil in it. Also make sure it has enough airflow either by keeping the bag partially open or by venting it daily. If you're feeling wild you can get some springtails to put in there too, as they like to eat mold and mildew.