I don't really see why a refugium would be used, unless they used that to put planting substrate in and planted the willows properly. However, seeing as it's just being used to get rid of green water, a temporary endeavor, I don't really think its necessary.
Refugiums are basically... tanks or containers used in conjunction with the tank, but are divided from it. Sometimes I'll see them in my LFS as those critter keepers that are sunken into the water holding something like a crayfish, that might hurt the fish inside the tank otherwise. Or like a breeding net, sort of. Not very good examples, heh. ^^"