A credenza is a piece of furniture popular in the late 19 th century used as a second work space beside a desk. It's modern name would be a TV console. The term " Moss covered, three handled family credenza" comes from the Dr. Seuss book "The Cat in the Hat".
Well, could a Grudunza (gredunza?) be part of a cradenza? Maybe it's one to be used outdoors, to store garden stuff in, that's why it's "moss-covered'.
First, it comes from the 1971 movie, not the book. Second, it's a grudunza, not a credenza. Ironically, the joke wouldn't make sense if it was a credenza, because there is such a thing as a credenza.