。
A shadow of anxiety flitted across her face.
Literal
Anxiety [possessive-の] shadow [subject-が] her face [object-を] swiftly crossed.
A literary sentence with vivid imagery. さっと is a mimetic adverb describing swift, fleeting motion — here the abstract 不安の影 ('shadow of anxiety') personifies the emotion as something that flashes across her face. よぎる means 'to cross, pass by' and is commonly used for fleeting expressions or thoughts.