List posts in a given category

You are already passing category in $args. so not need to use get_categories(). You can use below code.

   $args = array(
        'cat' => 5,
        'posts_per_page' => -1,
        'orderby' => 'title',
        'order' => 'ASC'
    );
    $custom_loop = new WP_Query($args);
    if ($custom_loop->have_posts()) {
       echo "<ul>";
        while ($custom_loop->have_posts()) {
            $custom_loop->the_post();
            echo '<li><a href="' . get_permalink() . '">' . get_the_title() . '</a></li>';
        }
       echo "</ul>";
   }
   wp_reset_query();