Ok, so you accidentally type in the root password before using su -.
Now you have to remember to delete it from your history file. This is somewhat non-obvious, because the history file is typically saved on successful exit.
So if you immediately do something like this.
$ tail -2 ~/.bash_history ssh somehost su -
There’s nothing suspicious in the history file.
So, log out, then log back in. Typically that means closing your ssh session, or terminal tab, and open a new session.
Now you see the password in the history file.
$ tail -2 ~/.bash_history su - ThisIsMyRootPassword
So fire up an editor and delete the line.