Also juk gai choy and variations (Chinese), small gai choy, and stickleaf mustard
The names listed are likely matches for the vegetables pictured according to a group of confused experts (myself included). But when you do see this ubiquitous Oriental market mustard, the bunch alongside will probably look different enough for you to wonder about its identity—as we all do. It may be darker or lighter romaine green, multi-branched or single-stalked, wide-stemmed or slim-stemmed, ribbed or not, leafier or less so. It is always smaller (juk means small) than dai gai choy (dai means large).