Cant say for sure because the photo's not in focus, but to me it looks like rust stains, quite common on UPVC window sills if you get metal shavings or filings left on the sills they will rust and stain the white plastic, if the staining has etched into the plastic then usually it's permanent and cant be removed.Hope this helps.
I'd just do a quick 'best effort' and leave whats left! But I am unprofessional apparantly!