How do i clean up a polyurethane stain from a composite deck?

Long story short, I moved into a new condo yesterday, had my interior floors screened, and the floor guys left noticeable stains on the shared front porch (composite decking).

I've called the floor guys to clean it up but

a) they aren't making it a high priority

b) I'm worried they'll do a bad job of cleaning and I'll have to finish it myself anyway.

I want to make sure this gets done well since my literal first introduction to my upstairs neighbor was him pointing out the stains (oops). I'd like to make a better first impression than that as a new neighbor:-/

What's the best way for me to clean up noticeable poly stains of the deck without doing more damage?