Use custom field as tag slug

first get tag slug from your saved meta key my-tag-slug in a variable

$tag_slug = (isset(get_post_meta(get_the_ID(), 'my-tag-slug', true))) ? get_post_meta(get_the_ID(), 'my-tag-slug', true) : null;

now query posts by $tag_slug replace showposts with posts_per_page

$args = array(
  'tag' => $tag_slug,
  'posts_per_page' => 5,
  'caller_get_posts' => 1
);
$my_query = new WP_Query($args);
if( $my_query->have_posts() ) {
  echo '5 recent Posts with tag1';
  while ($my_query->have_posts()) : $my_query->the_post(); ?>
    <p><a href="https://wordpress.stackexchange.com/questions/113547/<?php the_permalink() ?>" rel="bookmark" title="Permanent Link to <?php the_title_attribute(); ?>"><?php the_title(); ?></a></p>
   <?php
  endwhile;
} //if ($my_query)
wp_reset_query();