Umbriel appears darker in this image, but the two are actually fairly close in brightness. Notice the white patches near the top, and at about 1 o’clock and 7:30 on this image. More bright spots on Umbriel’s companion, Titania (image attribution here):
This moon is more spotted in white than either Ceres or Umbriel. The prevailing thought on the Uranian moons is cleaner ice dredged up by meteor impact. Or maybe water welling up from deep inside and freezing–the outer solar system equivalent of lava solidifying after a volcano erupts.