He's mildewstaned he's mouldystoned

Mildew Lisa (page 40), referring to Wagner's "Tristan und Isolde", the famous dying aria of which begins: "Mild und leise" (Mild and quiet).

"Stan" and "Stone" refers to Shaun (and to Joyce's brother Stanislaus), "stem" or "tree" to Shem (or James Himself). (And if you put Tree and Stone together, you get "Treestone" or "Tristan".)