Thanks @phatch !
As I recall, mei cai is dried.Yibin yacai, the fermented vegetable that flavors the meat topping, is made from the sprouting greens of one of dozens of cardamine species cultivated around China (cardamine belongs to the mustard family). In Yibin, a city in southern Sichuan province, yacai is double-fermented with sugar, giving it unique complexity. You can find "Yibin yacai" or "suimi yacai" ("loose rice sprout vegetable") sold in the pickle aisle of good Chinese supermarkets, or online, but if you need a substitute, look for one of its cousins. Mei cai or meigan cai is another fermented mustard green from southern China, and xue cai or xuelihong from Jiangnan is sweeter but also reasonable. Zhacai is another Sichuanese fermented mustard, but I'd consider it a last resort - it's typically spicy, which yacai isn't, and it's made from mustard stems rather than greens, which gives it an entirely different texture.
YACAI 芽菜
English Names: Dried Vegetable, Fermented Mustard, Preserved Mustard Stems
Made from: Mustard Greens
ZHACAI 榨菜
English Names: Sichuan Preserved Vegetable, Preserved Mustard
Made From: B. juncea subsp. tsatsai var. tumida
GANLANCAI 橄榄菜
English Names: PS Olive Vegetable, Preserved Mustard in Brine
Made from: Chinese Olive Greens and Mustard Greens
XUECAI 雪菜
English Names: Snow Vegetable, Pickled Cabbage, Preserved Pot Herb Mustard
Made from: Brassica juncea var. crispifolia
SUANCAI 酸菜
English Names: Sour Mustard, Pickled Mustard
Made from: Mustard Greens -or- Napa Cabbage
MEICAI 梅菜
English Names: Dried Mustard Leaf, Dried Fermented Greens, Dried and Pickled Greens
Made from: Small Mustard Greens
DATOUCAI 大头菜
English Names: Preserved Turnip, Preserved Radish
Made from: Turnip
Unavailable online, can sometimes be found in Chinese supermarkets under Cantonese name "Tai Tau Choy"
DONGCAI 冬菜
English Names: Tianjin Preserved Vegetable, Tianjin Winter Vegetable, Tianjin Salted Cabbage
Made from: Napa Cabbage
LAHPET
English Names: Cultured Tea Leaf, Pickled Tea
Made from: Tea Leaves