No, it will not be safe to move the mallard nest. If you do, the duck will undoubtedly desert the nest. In addition to being illegal to disturb a mallard nest, it would surely cause its loss. And should she desert, you should not try to hatch the eggs yourself. That, too, is illegal and futile because you cannot match the care of the mother. My suggestion is that you leave the nest where it is and hope that the construction will not disturb the ducks.