To turn it off go into "Settings". Under the "Personal" section click on "Accessibility", then "Dexterity and interaction", then "Assistant menu". At the top of the "Assistant menu" window turn the switch to "off. The blue dot should then disappear. Thank you so much, that blue circle was making me crazy!
Tap on the center of the circle and then on "Hide Controller". You probably inadvertently clicked on that option while the Zoom window was open. You can turn off Zoom altogether if you wish by going to Settings>General>Accessibility>Zoom = Off. Cheers,.