Hello,
Which model Mac is this?
It looks like either failing pixels, or debris behind the glass. Depending on the Mac model, the only way to fix it would be to replace the LCD display. There isn't any way to separate the glass from the LCD assembly, unless it is an older model iMac. But, that isn't recommended, because the LCD panel itself can easily be scratched, or additional debris could be introduced behind the glass without the proper tools and training.
Hope that helps,
C