Hi luisfernandoperu
When you add a html code to a custom module would be better to set default joomla editor to none because they can strip out html code
I added correct code now and as you can see it works fine
https://prnt.sc/py7291
If you need to edit it then first disable default editor from global configuration save it and then enable again editor
This is the correct code to add to it
<ul class="social-list">
<li><a href="#" title="Facebook" class="btn facebook"><span class="fa fa-facebook" aria-hidden="true"></span><span class="empty">empty</span></a></li>
<li><a href="#" title="Twitter" class="btn twitter"><span class="fa fa-twitter" aria-hidden="true"></span><span class="empty">empty</span></a></li>
<li><a href="#" title="Instagram" class="btn instagram"><span class="fa fa-instagram" aria-hidden="true"></span><span class="empty">empty</span></a></li>
<li><a href="#" title="Google plus" class="btn google-plus"><span class="fa fa-google" aria-hidden="true"></span><span class="empty">empty</span></a></li>
</ul>
Regards