If you enabled authentication for mongodb, then moadmin will fail to show the databases. This is because moadmin is not authenticated to list the databases. You might get the following error.
I am considering a scenario where I enabled authentication as said in the link here. In that case open your moadmin.php file and make the following change, so that it authenticates itself.
You need to edit MONGO_CONNECTION parameter.
Save and quit the file, and now try accessing the interface. It should work.