This may seem silly, but there are a lot of interesting implications to the answer.
Does the alien have a concept of color? We learn about biology. If we know about their star and habitable zone, we can surmise a lot about not just them but their environment.
We learn something about their aesthetics. Maybe we can't understand them or they are relatable to us.
The point is, ask a question which can provide lots of other info.
(Though there's a fun short story from way back that ends with aliens who have FTL travel wanting to buy our high-reliability computing/robotics tech, because they've never developed any for themselves :-)
Hopefully said aliens are visual in nature and understand spacing.
If they do, this works also: ○
"I figured you'd be an alien, babe, because your ass it out of this world".
The point is, if you are a stud and the girl is attracted to you, almost anything works, even borderline sexist stuff.
If not, then yeah, you need a lot more work that some pickup line. But it's not like they rejected you because you used a lame line.