A girl pretended having a piece of food in her hand and pointed it to a Barbary Macaque in Gibraltar. As a response he made this facial expression to her. What does this mean?
It looks like it is trying too threat.
That is a threat face. Barbary macaque threat faces often appear with a brow raise, lowered head, and an o-shaped mouth, sometimes with and sometimes without a vocalization. Given the context you described it is not surprising the girl received a threat. *Based on personal research experience