Change posgresql 9.3 slave roll password
the person who set it up left the password the same as the password
  for the super user account on our other server


If the other server is the master, it's normal because having different passwords in the replicated database is not possible.
PostgreSQL replicates everything from the master, including user accounts and their passwords. There's no way to divert that temporarily.

If someone needs to be superuser on the secondary without knowing this password, a different authentication method should be used on this server, such as Certificate Authentication, or external methods (ldap, radius...) . As a configuration file, the pg_hba.conf can be different on the slave.