Geese eat water plants if they are available and they might eat your moss balls, but they don't have any way of straining the algae out of the water,although if you had fish in there to eat the algae, they might enjoy them.
I change their water at least once a day and try to change their pools ( childrens plastic pools) every couple of days. I could be wrong, but I'm with you and don't want them drinking water they've fouled.