Hi Paul.
Try
lightening settings from Windows Control Panel>Printers and Devices>right-click XEROX>settings.
My CMYK and mono printers are SAMSUNG, last one was HP.
The menu screenshot should hopefully be similar to Xerox.
As only the XEROX is doing this, I suspect your XEROX specific user settings in Windows are set
too dark.
The signature was probably done on paper, which is fibrous when magnified.
When scanned, the slight shadows between the fibers are converted to random white and slightly off-white pixels.
To the human eye it appears white, but when darkened, the not pure white pixels are visible, creating noise.
Besides setting darkness to
normal or
light, you could also try setting ECO to
on. If the xerox
has economy printing.