Denpa Onna to Seishun Otoko, Gokukoku no Brynhildr Recommendations
Both have a main character who believes in the existence of aliens, but in Denpa the main character thinks she's an alien, while in Brynhildr he just thinks they exist. Both have the alien aspect, but actually have little to do with aliens.
Both animes focus on a blue haired girl who likes stargazing and has an obsession with aliens. In both of them, the male lead ends up looking after after her, and they have a fairly close relationship. There is also other girls who want to get with the MC. They both have somewhat similar art styles as well.
Other than that they're completely different though.