The password history for a target account shows EXPPWD as the user by whom the password was changed.
There is no EXPPWD entry in the list of users on the Users > Manage Users page.
CA Privileged Access Manager (PAM) Credential Management
EXPPWD is an internal PAM user that is used by the Expired Password processor to update expired passwords according to settings in the password composition policy associated with the target application that the target account belongs to.
For details on how the expired password processor works, see article Password Expiration configuration in PAM.