You could add this to your public_html .htaccess file:
RewriteEngine On
RewriteRule ^/?$ /joomla/
That way you can still go to the pages in your public_html folder but this will redirect you to the index file of the joomla folder:
mysite.heliohost.org
You might need to write it this way, adding this [R=301,L] for your images to work?
RewriteEngine On
RewriteRule ^/?$ /joomla/ [R=301,L]
the other way the browser doesn't know it's been redirected.