Currently that is not possible.
Credential alarms are based on an event and not on a rule. You can exclude alarms based on a rule or you can exclude alarms based on severity, but excluding only credential alarms is not possible. Details how to exclude alarms based on a rule are here.
But you can exclude them on the PagerDuty side, that they will not be forwarded to your pager. Please contact PagerDuty support to get further details.
