How to remove clear oval from iPad screen?

You appear to have enabled the iPadOS Window Zoom function.

The iPad User Guide is a rich (and often overlooked) source of information about iPad and iPadOS. Here is a direct link to the relevant section of the Guide:

Zoom in on the iPad screen - Apple Support

You can control the many Zoom features from iPad settings. To disable all Zoom functions:

Settings > Accessibility > Zoom > Zoom - set to OFF

Can you post a screenshot? This will greatly help contributors to understand what you see - and in so doing assist with providing appropriate help or guidance:

Take a screenshot on iPad - Apple Support

You can post a screenshot (redacted if necessary, using markup tools, to obscure any personal or sensitive information) using the image button found at the bottom of the message editing window:

A screenshot can also help to determine if you have a hardware or software issue. When reviewing the screenshot, if the screen artefact is not visible 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 artefact is 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

If the test indicates a potential hardware fault, a visit to the Genius Bar at your local Apple Store is recommended - where the technicians will be able to test and assess your iPad. This will likely inform your next steps:

Genius Bar Reservation and Apple Support Options - Apple

iPad Repair – Official Apple Support

If instead the test suggests a software issue, try a forced-restart - as this will often clear an isolated software issue:

Force restart iPad (models with Face ID or with Touch ID in the top button)

If your iPad doesn’t have the Home button, do the following:

1. Press and quickly release the volume button nearest to the top button.
2. Press and quickly release the volume button farthest from the top button.
3. Press and hold the top button.
4. When the Apple logo appears, release the top button.

If the problem persists after performing the restart, it may be necessary to reset your iPad settings. While this reset will not cause any loss of data, as a precaution, it is always wise to ensure that you have a current iCloud or iTunes backup before performing the reset:

How to back up your iPhone, iPad, and iPod touch - Apple Support

When you are happy to proceed, you can perform the reset from iPad settings:

Settings > General > Transfer or Reset iPad > Reset > Reset All Settings

If the problem still doesn’t clear, the next step is to reset the iPad to factory settings:

Restore your iPhone, iPad, or iPod to factory settings - Apple Support

Otherwise, you would be best advised to seek assistance directly from Apple Support. You can contact the Support Team using the Support link at top-right or bottom-left of this page. Alternatively and perhaps easier, you can initiate and manage your support cases using the excellent Apple Support App. If not already installed on your iPad, the App can be downloaded from the App Store:

https://apps.apple.com/app/apple-support/id1130498044

You might also visit the Genius Bar at your local Apple Store or Apple Authorised Service Provider (AASP) - where the technicians will be able to test and assess your iPad - and you can explore your available options:

Genius Bar Reservation and Apple Support Options - Apple

How to remove oval on iPad?