how to get title to custom tags page

To expand your conditional to check for display of one of your terms, you can do this:

if ( is_page() ) {
    the_title();
} elseif ( is_tag() ) { 
    single_tag_title();
} elseif ( is_tax( 'mtags' ); ) { 
    single_term_title('More Tags: ');
}

However, if you use your theme’s custom taxonomy template for this taxonomy, taxonomy-mtags.php, then you can just do something like:

<h1 class="page-title">
    <?php single_term_title('More Tags: '); ?>
</h1>