Sorry for answering so late.
The manual isn’t clear enough about the virtual host. You don’t have to create it manually with virtualhosts module. The virtual host is created by nethserver-mattermost module once you specify a fqdn (different than the one used as the server hostname). The auto-configured virtual host is considered internal to the module and will not be shown as part of user’s configurable virtual hosts (virtualhosts module).
mattermost is configured at port 5432 while rh-postgresql94-postgresql is at port 55432. Easily mistaken due to their resemblance.