Just as you want to prevent mildew from forming on your tent, the same can be said for your footprint. The best way to allow mildew to form is to store a material that is damp for 24 hours. What mildew can do is permanently damage the waterproof coatings on the material as it can cause it to separate from the fabric. The problem with this is after it has formed, the stains will not go away. Thankfully, however, mildew is something that is very easy to avoid. It can’t grow without moisture so granted you ensure that the tarp is dry before you store it, you will be fine. Really, just do not get too lazy when you get home from a long vacation.