running pg_dumpall makes a backup of the cluster "main", but I have another cluster "custom". How do I make pg_dumpall produce a backup of "custom"?
Every cluster is served by a different Postgres instance, listening on different port and/or using different unix domain socket (if there are different instances running on one machine).
So you need to pass different value for the port:
-p port --port=port Specifies the TCP port or local Unix domain socket file extension on which the server is listening for connections. Defaults to the PGPORT environment variable, if set, or a compiled-in default.