Hearing Zach's moan made Fred want to speed it up, make it hotter, but it also made him notice that they were still outside, in a public place. Though he didn't really care who saw them, it really wouldn't do to be interrupted by other people's complaints if they went too much further.
Fred gave a low chuckle as Zach arched up and the chocolate spread only further down. He figured he might as well just unbutton the whole shirt seeing how low it was heading, and did so. As he shifted to give himself a better angle at the lower half of Zach's torso, he put a hand over the waistline of Zach's trousers to prevent any chocolate from dripping down that far. Though there wasn't any down that far it was still dripping and Zach wasn't helping to slow it down - but Fred had no intention of letting it get that low anyways. He started again on the lowest drip - which was beginning to fall onto Zach's abs now - and made his way across Zach's chest with his tongue again, this time thoroughly cleaning.