I thought I could replace the text in the footer via a function, but maybe that is not possible for the Poseidon theme. I had something with remove_action and such, but it did not work before I lost my patience. But ofc there is something you can always do when using WordPress: Override the footer.php file via your child theme (you are not trying to mess up the original theme, are you?!).
(Step 1) Copy the footer.php from wp-content/themes/poseidon to your child themes directory on the webserver.
(Step 2) Remove or comment out the line that contains the following:
<?php do_action( 'poseidon_footer_text' ); ?>
(Step 3) Insert your text in its place inside the div that surrounds it.
(Result) The result should look something like this:
Copyright Jo · Theme by <span class="credit-link"><a href="https://themezee.com/themes/poseidon/" target="_blank" rel="nofollow">ThemeZee</a></span>.
And that’s it. While this is for the Poseidon theme, you can easily adapt this for your own theme. Copy the footer.php to your child theme, edit the relevant parts and enjoy :-)