Hi,
Show me your workflow to task binding for this task which you shown in your screenshot.
I think you are displaying workflow initiator user id (_WF_INITIATOR) in your inbox work item body text. And you have binded the same from your workflow container to the above task continaer (WF_INITIATOR container).
_WF_INITIATOR comes filled with 'US' at the start automatically by SAP . If you just want the workflow initiators user id to be displayed on body text, use container _WORKITEM.CreatedByUser.ID in the workflow to task binding for above task container element WF_INITIATOR. it will pass only userid withoutt 'US'.
Regards,
Ibrahim