Get css class of menu item in custom menu structure

What you’re getting is an array so you need to implode() them – the Walker_Nav_Menu has something like this:

$class_names = esc_attr( implode( ' ', apply_filters( 'nav_menu_css_class', array_filter( $classes ), $item ) ) );

So you can modify your foreach to implode them in a similar fashion

$class = esc_attr( implode( ' ', apply_filters( 'nav_menu_css_class', array_filter( $menu_item->classes ), $menu_item) ) );

We still want to make sure that the nav_menu_css_class gets applied in case modifications has been made form that hook.