Mold is definitely not good and indicates a major problem in the wash routine or storage. We do not have a warranty policy for this as it is not a manufacturing defect.
So first of all, a hack to help reduce staining is that if you have some Borax, mix it with a bit of water to create a paste and scrub the spots with a toothbrush. It should considerably helped lighten the spots, although it won't fully remove them. You can see below an example of Before the Borax scrub vs After.
The Borax scrub won't kill the mold, so you will also need to add 1/2 cup of bleach to your next wash on cold with detergent, you can also add towels or anything bleach safe to bulk with for agitation. This will kill the mold spores and prevent the mold from spreading. You can follow the bleach wash with a warm water wash with detergent to remove any residual bleach and you'll be good to go!
If stains remain after the bleach soak, the next step you can try is an OxiClean paste. OxiClean powder is an oxygen bleach made up of washing soda and a solid form of hydrogen peroxide. When combined with water and made into a paste, it’s very effective at removing stains. Mix OxiClean and water into a paste consistency, and apply to paste to the stained area. (Extended direct contact with OxiClean may cause skin irritation, so use gloves or a spoon to apply the paste.) Let sit for 10 – 30 minutes, then wash.
If there is still a bit of stained spots after, they're simply cosmetic, but putting your PUL in the sun may help lighten them further but beware of extreme heat/ direct sun for long periods because it can cause the PUL to delaminate, which is also not covered under warranty.