If you have cleaned the screen (with a damp microfiber cloth, preferably with a screen cleaning solution if you have one) and this persists then it is likely the matte screen surface. It is quite high-haze ('thick' or 'grainy' as some would describe it) on this and similar monitors and that can give the image an appearance of something being 'smeared' over the screen. Do you find this more noticeable on white or particularly light colours? Perhaps if you move this window across the screen for example?
Could you guys let me know the severity of the anti-glare coating's 'smeared' or 'grainy finish' look on your XL2730Z? Mine is more noticeable than I would expect of a monitor at this price, but it's the first 'high-end' monitor I have had so I have nothing to compare it to. I'm starting to consider replacing it with the MG279Q or doing an RMA.
Thanks for the help.