The moonset view came from one sol; Justin extended the mosaic with some images taken a previous sol. "The sol 610 frames were adjusted to match the color of the Sol 613 image. As these additional frames were in the opposite direction of the Sun, very few shadows are present, ideal for matching the post-sunset lighting conditions of the sol 613 image," he writes.
A bit of Phobos trivia: Curiosity's view here is to the east, but this is indeed a moonset, not a moonrise. Phobos orbits so close to Mars that it moves around Mars faster than Mars rotates, and consequently it appears to rise in the west and set in the east!
Thanks, Justin, for finding and sharing this gem from Curiosity's archives!