As far as I know there is no way to display only a particular module inside the IFRAME. If you want to display a module from a different page on the same portal, you can do it by adding that module to the page - first select Existing Module radio button, then select the page from a dropdown and, finally, select a module to be added.

WARNING: it will act as an instance of that module - so, changes to the module content can be done on either instance and will affect all. Deleting the module will not, however, delete it from all pages where such instance was added.