One of the primary purposes of the camera is to confirm that the high gain antenna has been deployed properly. This requires that both the flat reflector (electrically "parabolic") and the antenna feed at the parabola's focus have both been deployed from the cubesat correctly.
The black item protruding is near the surface of the reflector and is tilted "forward" from the surface normal, allowing it a view of a large area of the patterned reflector as well as the tip of the High Gain Antenna Feed. The camera field would have been carefully calibrated and so the image of the reflector's pattern provides good data on any deviations in the reflector's deployed shape and angle.
The slightly offset location corresponds nicely with the offset of the antenna feed to the lower left in the image in the question, as well as the newer image of Mars shown below.
This has the right size, color, position, and orientation to be the camera, and nothing else fits those constraints.
below x2: Source: Planetary Society's Emily Emily Lakdawalla writes MarCO: CubeSats to Mars!
below: from Farewell to Mars found in this question
UPDATE! I just found these, which confirms it: https://www.jpl.nasa.gov/news/press_kits/insight/launch/appendix/mars-cube-one/