You can hide it by changing the following code in the PHP file (Editor manager) :
Edit line 302 of file /wp-content/plugins/wp_mail_smtp/wp_mail_smtp.php:
<td><input name="smtp_pass" type="password" id="smtp_pass" value="<?php print(get_option('smtp_pass')); ?>" size="40" class="code" /></td>
This answer was given 6 months ago.