Adding more detail to what @owldesign said:
Here is the full list of WP icons one can use to changed the icon of an option page.
When adding a Option page through your functions.php:
acf_add_options_page( array(
'page_title' => 'Home Page Sliders',
'menu_title' => 'Sliders',
'menu_slug' => 'home-page-sliders',
'capability' => 'edit_posts',
'icon_url' => 'dashicons-images-alt2', // Add this line and replace the second inverted commas with class of the icon you like
'position' => 7
) );
