this looks like you have little polygons with a certain material applied - rather than filling a white color, try filling with the "Flat Color" material. This is the base material in Zbrush and will remove any material assignments to any polygons you may have accidentally added (usually this is caused by using something…
It's that feel when you finish unwrapping something, and after a day of playing tetris with polygons, you get something that's efficient and easy to use.