。
The foam vanished in no time.
Literal
Foam [topic-は] visibly-quickly disappeared
みるみる is a 擬態語 derived from 見る (to see) meaning 'before one's eyes, visibly, rapidly' — as if the change happens faster than you can watch. Commonly pairs with verbs of change or disappearance: みるみる小さくなる, みるみる変わる.