Mildew and stains on paint chip guard (clear coat)?

Dang, was that clearcoated? If you can't get it off, you may just want to put down a protector, I have these on our 2017 and 2021 Prius'...

 

Yes. It is "upsell" junk.
When mine was in the body shop for a minor "ding", I had them take all of it OFF.
I don't think they even charged me to do that.

 

If the chemicals used were too harsh on plastics, it could destroy it. If removing it, you might need to heat it up as you peel it off the bumper.

Useless options are always a way the dealer can make more money. Especially in today's crazy hot market, every car is loaded up with crazy options. Even if you buy the car at MSRP, the useless options will make the car way more expensive than it should be

 

I recommend obtaining a product called Bestine (heptane), available on Amazon (expensive) or at art supply stores (less expensive...) and places like Staples. It is an organic solvent which will NOT affect paint or almost any plastic (including your problem). It will only attack the organic solvent holding the plastic to your bumper. Just work it in a bit at a time with a dropper or a paper towel (it's quite volatile, so you'll have to keep adding it). I've used it for about 40 years to good effect.

 

This makes me think about those small zones of plastic overlay, on the rear side door openings.

one time I parked under a tree that was dripping pitch. What worked, albeit with a fair bit of elbow-grease, was a rag soaked (and wrung out) with kerosene, plus a dab of car polish.

 

No need to remove it. Just get a spray paint can of white plasti-dip paint (aka: peelable paint) and spray it on... It is a very thick fast drying paint and takes touch ups spray on damage really well. It also peels off entirely if you want to put on a new coat.