Adding Content on All Posts Under a Specific Tag

Hello there,

I’m trying to add texts on all posts under a specific tag but I can’t get it into work.
Here’s my code snippet derived from one of the tutorials in the internet:

<?php if (is_tag('greek-mythology')) { ?>
<span>Greek</span>
<?php } ?>

Based from the code above, I want to add the work “greek” on all posts under the “Greek Mythology” tag. Any idea how to make it work? Thanks.

By the way, I added that code on my single.php file before the <?php the_content(); ?>.

I’m no programmer and a code snippet as a solution would be a big help.