Maybe these particular sculptures weren’t exactly rock solid. If you know what I mean.
I’ve picked up some pretty soft poops from time to time. You’ve just got to grit your teeth and do it.
Use what comes to hand. A spatula and a plastic bag, for instance, sponges and paper towels. They could afford to throw the spatula out if they didn’t want to clean it—a lot cheaper than making an unscheduled landing.