That is not some sort of weird crescent on the Saturn moon Iapetus, The dark area is a deposit of dark material and is on the leading side as the moon orbits Saturn. The dark material is said to be a reddish brown color and has a low albedo, reflecting only about four percent of the light that hits it which I think is about the same as charcoal.
NASA – Iapetus is a world of contrast, with light and dark regions fitting together like cosmic puzzle pieces.
Cassini Regio on Iapetus (914 miles or 1,471 kilometers across) is covered in a layer of dark, dusty material creating a stark contrast to the much brighter region that surrounds it. This leads to the moon’s distinctive, two-toned appearance. To learn more about the cause of the contrast between regions, see PIA06166.
This view looks toward Saturn-facing hemisphere of Iapetus. North on Iapetus is up and rotated 20 degrees to the right. The image was taken in visible light with the Cassini spacecraft narrow-angle camera on March 11, 2017.
The view was obtained at a distance of approximately 1.6 million miles (2.6 million kilometers) from Iapetus. Image scale is 9 miles (15 kilometers) per pixel.
Image: NASA/JPL-Caltech/Space Science Institute