。
She had a look on her face as if she had seen a ghost.
Literal
[topic-は] just-as ghost [object-を] saw-[such-as-ような] face-look [object-を] was-doing.
まるで~ような is the canonical simile construction — 'just like, exactly as if' — paired with 見たような ('having seen') gives a counterfactual or hypothetical comparison. まるで sets up the simile with high expressive force: 'totally as if [it were].' 顔つき ('face-look, expression') is more concrete than 表情 — it refers to the molded look of the face rather than the transient expression. 顔つきをする ('have a [type of] face') is a stock collocation. 'Seeing a ghost' is a near-universal simile for shocked terror, working in Japanese as in English.