How do I cancel my storage plan on my IPad Pro?

If you have already done the downgrade, that would be normal.

Also see the following from Downgrade your iCloud storage - Apple Support

When you downgrade your iCloud storage plan, the storage decrease takes effect after your current monthly or annual subscription ends.

If still in doubt, please reply with a screenshot of the window (or look carefully at all of the wording in the window).

The fact that it says "Expires on" confirms that it has been cancelled. I don't have the exact wording but I think it is "Renews on" with the date when it is an active subscription.

Cancel by downgrading to the free 5GB plan. See instructions in: https://support.apple.com/en-us/HT207594