As a professional photographer, you of course have no legal rights to the image if it was taken in a public space. There are some exceptions, but assuming there's no trespass, no nudity, no overt agression, and a few other obvious caveats, then typically you have to accept it.
If you approach the photographer I'm sure 99.9% of the time they'd be happy to either explain the project they're working on & /or delete said image as you wish.
The tone of the conversation would be entirely dictated by you. If you ask rather than instruct from the outset I feel sure there's nothing malicious intended – you might even get a copy of said picture for your troubles (you might even appreciate it in years to come!).
Tim