。
She's quite pretty but looks unhealthy.
Literal
She [topic-は] quite pretty [but-が] unhealthy seems.
不健康 ('unhealthy') is built with the negative-prefix 不 attached to 健康 ('health'); the same 不 gives 不便 ('inconvenient'), 不安 ('anxious'), 不可能 ('impossible'). Combined with the appearance-evidential ~そう, 不健康そう describes how she looks rather than her actual condition. The contrast structure なかなかきれいだが不健康そう ('quite pretty, but looks unhealthy') is a classic Japanese hedge — preserving the compliment while qualifying it, characteristic of indirect Japanese comment-making.