I’ve heard that corrosion on the titanium finished p99s is not uncommon, which is odd because titanium is supposed to be highly corrosion resistant. I’d love a detailed explanation from someone who knows their stuff what exactly is going on here.
As for what you can do with this, your best option might just be a can of Nevr-dull from Home Depot for 6 dollars. It’s supposed to be a rust remover that’s safe on all metals. If you’re willing to spend the money on a more permanent solution, sending it off to get NP3+ coated will keep the look while actually being an amazing coating. Might cost more than the gun is worth though.

 

I found this post in the Walther forum from 2014. Doesn't mean it's true but people have mentioned it before:
"However, the titanium version is known to speckle black over time. I would not purchase a titanium based on this."

https://www.waltherforums.com/threads/just-how-rare-valuable-is-my-titanium-coated-p99-9mm.39881/

 

I haven't had that issue, but I'd proceed with the least drastic and work my way on up. The cleaner suggested seems like a good first start. Perhaps moving up to a polish like Mother's Mag wheel or similar, but these are mildly abrasive. Coating would be my last option. Best of luck.

 

I don't have any firsthand experience, but I'm kind of fascinated. Does it seem like the corrosion is starting from the top down (maybe a smooth and continuous surface), or coming up from underneath (maybe pitting and/or titanium flaking off)? Titanium is so noble on the galvanic series that it turns most other metals into sacrificial anodes and corrodes them. If there was even the tiniest pinhole through the titanium down to the steel, with some finger salt and not enough oil in it (and many oils evaporate over time), the salt could have sucked moisture out of the air and corroded the steel from under the titanium. Where the corrosion is coming from affects your options.