I don’t remember exactly where the apple is, but it isn’t available until after you return from the Moonlight Abductor and have access to the gallery, it is not in the gallery.
Sorry, but we don't really want to spoil how to find all of the apples. But there are a few clues that we tell people:
Nine apples are outside, one apple is inside. All of the outside apples are on or near trees and plants. The inside apple is NOT in the 'underground maze'
