It will respawn on its locations. If killed twice (2x), it will respawn after 12 hours.

How often do Hilichurls Respawn?

If the first one is defeated but not the second one, 12 hours after the first kill the spawn locations and count will reset. If the two Unusual Hilichurls are not found, they will respawn in a different location after some time (exact time is currently unknown; probably 12 hours).

How long do Genshin flowers take to Respawn?

Remember, the flowers will refresh around every two to three real-world days.