Hello! I just got my moto g about an hour ago and am noticing the same issue but less pronounced.
Its not over all the screen it mostly is found on the borders of the screen but is not consistent. It doesn't show up on the boot screens and appears in some pictures only I.e screenshots or images while web browsing.
Another curious thing is that it mostly affects darker colors., black almost always being the worst offender.
I have no downloaded apps on the phone, I recieved it, downloaded an update,proceeded to download apps and such but afterwards gave it a factory reset to see if it still appeared and it did.
Hopefully I can get some help!
Thanks in advance!