。
She's cold to me.
Literal
She [topic-は] me [to-に] cold-hearted is.
冷淡 ('cold, indifferent, apathetic') is a な-adjective combining 冷 ('cold') + 淡 ('faint, light'); together, the sense is 'emotionally faint, distant, not warmly engaged.' Distinct from the physical 冷たい ('cold to the touch'), which can also be used metaphorically for personality but in a slightly different shade. The に here marks the speaker as the target/direction of the coldness — 私に冷淡 ('cold toward me'). Useful sentence for relationship vocabulary: 冷淡 captures emotional unavailability with a literary flavor.