what is the black line middle bottom of my new iPad Pro and How do you turn it off it is sometimes white

Those are visual swipe indicators bars to let users know where to swipe to initiate certain swiping gesture features of iPadOS.

There are a few of these present throughout iPadOS.

They are part of the iPadOS UI.

They cannot be switched/toggled off.

The swipe indicator near the bottom of the iPad's screen under the Dock indicates if you do a short swipe upward, you can bring up the Dock within an app or a short swipe up to return to the iPadOS Home screen ( saves wear and tear on iPad’s Home Button ) or a longer swipe upward from the bottom of the iPad’s screen toward the center of the iPad’s screen to invoke the iOS App Switcher.

It’s just a visual aid as to where to swipe up to close the window/app. Not sure if they can be turned off

That shows you where to swipe upwards on the screen, in replacement of the home button. You cannot turn it off.