You can touch and hold that grey floating menu button to move it somewhere else on the screen.
To remove it entirely, you could turn Assistive Touch off in the same way you turned it on:
You can ask Siri to close (or open) Assistive Touch .. or, if set-up, you can use Accessibility shortcut to close it,
pr go to: Settings > Accessibility > Touch > Assistive Touch > Tap to turn it off (or on).
Apple Support's YouTube video below may be helpful with the above, and has other tips as well:
https://www.youtube.com/watch?v=_j4-NTg3QsE