Its called banding you just need to add a little noise to it.
depending on what version of motion you are using changes how to fix it.
I am on 5 so i do this
In stylise drop the add noise on to the background. then in the inspector change the noise type to blue noise (reduce banding). turn off auto animate and make the noise monochrome and your done.
In other versions of motion i think there is a filter called reduce banding.
Stu