Microfoam
What is microfoam in coffee?
Microfoam is milk steamed so that the air in it forms very small, even bubbles, giving a silky, glossy texture rather than a stiff, bubbly froth. A flat white is finished with a thin layer of it, about 6 mm, blended with the espresso's crema.
Tasting Table describes the microfoam on a flat white as less bubbly than the foam on a latte or cappuccino, and recommends steaming with the wand just below the surface at a slight angle so the milk swirls, until it’s warm, silky and slightly shiny. A cappuccino uses a thicker, denser foam on top.