The first thing to do is get the car and seat as dry and clean as possible. Mildew is a mold that thrives in warm, moist areas but still needs some food source to grow. If you can cut off one or more of its triggers you can stop it from spreading. Open the windows on a sunny day and let the car dry out and air out. Use a broom and vacuum and brush off as much of the powdery villain as you can and while you're there, give the seats a good cleaning as well. Crumbs, spilled food, and sweat are all yummy treats for the mildew that are found in just about everyone's car upholstery! Once you get all the dusty bits off the seat throw out the vacuum bag or it will begin to grow in there and stink up your house! Then, make a nice bubbly solution of detergent and with a well wrung out sponge or rag, scrub the seat covers. You don't want to soak the seat padding because that will just feed the mildew more moisture. Clean it as well as you can and then, if possible, let the sunshine on the seat to dry it and hopefully, fade the stains. Often, mildew can be washed out of fabrics but the black stains will remain. Now that you've gotten rid of the active outbreak and the factors that encourage it, you can try to get rid of the stain. Spot test any solutions first to be sure you won't make more of a mess if it's not colorfast! Bleach is the best first strike, but clearly will strip the color out of most fabrics. Instead, try a mixture of salt and lemon juice, spread onto the stain and then position the car so the sun is shining on the mixture to help its natural bleaching action. An oxygen type bleach (like Oxyclean) might work and is worth a try if the lemon juice doesn't work. Mildew is a nasty little mold and you may be left with permanent discoloration in the fabric. While mildew will eat through fabrics and destroy them if left untreated, the leftover black specks on clean fabric where the mildew has been killed won't hurt anything but your aesthetics!. If you've addressed the causes, it shouldn't come back. However, if your car is prone to that sour damp smell and doesn't seem to get dry, and the mildew comes back, you may have a hole in a wheel well or in the body of the car that's allowing water in under the carpet. An auto body shop can help find and seal the hole.