I think your best would honestly be to just leave it. You'll likely end up making it worse. Live with the cosmetic mark. I bought some PMC Fact.12 second hand and they have a couple of minor marks and small bits that aren't cosmetically perfect.
These things give your stuff character and even though its heart breaking to get some marks on things (I am really really bad for obsessing about this stuff - as my wife will attest to) I'm learning to get on with things.
So long as they sound amazing thats the main thing. Also, if you really wanted to you could get them re-finished or even wrapped but for that kind of mark its really not worth it. I find if you have that option in the back of your mind it helps you mentally deal with the cosmetic damage better.
I also had some gloss black pmc twenty.23 a while ago and they were littered with marks like that (second hand again). Annoying but I coped.
You have my full sympathy. I have a Kef Subwoofer that was perfect for 15 years. Looked like a Donald Judd artwork and then my 2 year old daughter put a ding in it. If you are obsessive-compulsive, like me, it will bug you to fix it and I second the mental aerobics of assuring yourself you will, one day, re-coat it professionally and then you'll probably find yourself just living with it. Having said that, if you really want to fix it yourself, contact Monitor Audio for advice. You need to identify what polymer or lacquer it is before investigating how to fill and polish it. There is always the chance you make it worse and a bigger eyesore than it currently is. You also need to be prepared to do the whole side or the whole speaker (looks like the edges are rounded) to integrate the repaired and polished section with the rest of it. I obviously don't have an immediate answer for you, but the manufacturer should be able to advise of the type of material that is coating the speaker, at the very least. Note: Different polishing compounds are designed for different surfaces - get that part wrong and you'll create some serious damage.
As others have said though unsightly it might best to leave it.
BUT depending out deep the scatch is its actually a very simple fix and only takes a little bit of elbow grease with good quality car detailing products (not turtle wax it will be abrasive). And by the looks of it , its only the lacquer that actually scratched.
If unsure take it to detailing garage for a quote or for some advice
Don't go too crazy the lacquer is probably pretty thin
Test on a hidden area, used this myself for years on cars.
Monitor Audio tend to have a good number of paint layers and the do recommend products like this on their Piano black finish. I did check with them a few years ago.
Try it by hand with a microfibre cloth.