You are allowed to say:
You look as if you have seen a ghost
Unless you want so specify a specific ghost e.g. in a haunted house that is known for a specific ghost that haunts that house, then you can use the below (again credits to @KateBunting)
You look as if you have seen the ghost
Using the below might be a bit odd
You look as if you had seen a ghost
"have seen" is present perfect tense and you can use it, and "had seen" is past perfect tense. This is because one is describing the person's current appearance. Using "had" can be odd after "look" (credits to @MichaelHarvey and @JamesK for clarification!)
Below are examples of "have seen" and "had seen" (credits to @MichaelHarvey)
Hey Joe! What's the matter? You look like [or/if] [you've/you] have just seen a ghost!
When I saw Joe last night, he looked [like/as] if he had just seen a ghost.