Final answer:
String and woodwind instruments are the performing forces heard in Schoenberg's Der Mondfleck.
Step-by-step explanation:
The performing forces heard in Schoenberg's Der Mondfleck, which is part of the song cycle Pierrot Lunaire, are string instruments and woodwind instruments. Specifically, the ensemble includes a flute (doubling on piccolo), clarinet (doubling on bass clarinet), violin (doubling on viola), cello, and piano. This set of instruments corresponds to the chamber music tradition of the early 20th century, where the interplay between strings and woodwinds was a common feature. While not specifically brass or percussion instruments, the ensemble provides a rich tapestry of sounds that characterize Schoenberg's expressionist composition.