that moon is Phobos, spied in the Martian dusk by Curiosity's Mastcam-100.

NASA / JPL / MSSS / Emily Lakdawalla

Phobos seen from the Martian surface, Curiosity sol 45 (September 21, 2012)

Just after sunset local solar time, Curiosity turned its narrow-angle Mastcam-100 toward the sky to image a crescent Phobos. This version is a slightly processed version of a raw image (blemishes and hot pixels have been cleaned up); it will likely be replaced later with an official release.