it should appear after completing most of the other events in the game
Viewing post in The Spirits of Sabil - NSFW Version (+18) comments
If you didn't call the Moki when first meeting them, they won't know you're attracted to them, and you won't be able to suggest having sex later in that save. You will be firmly platonic friends.
Spoilers below
If you did let the Moki know you were interested, you could proposition them for sex after they came back to announce that Ori successfully saved their village, and they would reciprocate your attraction and agree.