Are you using a screen protector? If so, this may be the cause; if not, then you may have a defective iPad screen.
A reliable method to confirm a screen or other hardware fault is to take a screenshot when the fault is evident - and then review the captured screenshot image...
When reviewed, if the patches are not present in the screenshot image, then you have demonstrated that you have a hardware fault that, sooner or later, may become sufficiently severe that repair or replacement of the iPad are the only options. If the patches are visible in the screenshot, the something other than hardware will be the cause.
Note: When reviewing the image, rotating the iPad screen orientation by 90°, from Portrait to Landscape, will ensure that the image does not coincide with screen defects if present.
Take a screenshot on your iPad – Apple Support
As indicated by Lobsterghost1, if a screen hardware fault is suspected, you would be best advised to seek a assistance from the Genius Bar at your local Apple Store - where the iPad can be properly assessed.
Genius Bar Reservation and Apple Support Options - Apple
If purchased new, you’ll have benefit of at least Apple’s limited one-year AppleCare warranty. If purchased the optional AppleCare+ Plan, your iPad will be covered for repair or replacement in addition to your standard warranty cover.