In the Customizer navigation settings you can chose whether or not to display a close button in an off-canvas template. As off-canvas templates are often used in combination with burger icons, a close button is often not necessary as the burger icon itself acts as a close button. However, on mobile screens it may happen that the close button in your template will be forced in order to prevent the menu from not becoming unusable.
You can easily change this with the following CSS snippet:
#mainmenuoff .hamburger {
display: none;
}
