ACF object in wordpress menu-walker
You have to pass the ID, associated : you can get it into $item->ID, you can know the type of this item print_r($item->type); (taxonomy, page, etc..).
Then :
$output .= '<ul id="about" class="dropDown">'. if(get_field('info_block', $id)): while(has_sub_field('info_block', $id)): .' <li><a href="#">'. get_sub_field('info_block_header', $id) .'</a></li>'. endwhile; endif; . </ul>';