Comment 1 for bug 1694223

As you discovered, the default credentials are changed during installation per best practice.

Relations get given their own private credentials when connected.

If you need to use Cassandra without relations to clients or need administrative access, you can find the superuser credentials on any unit in ~root/.cassandra/cqlshrc

You can also 'juju ssh' into any of the units and run cqlsh, which will connect as an administrative user by default. From here, you can create accounts with whatever password you like per Cassandra documentation.

I'll leave this bug open, as it can be better documented.