When referencing a user account in a Nintex Workflow a "i:0#.w|" tag is being added at the start of the username.
All SharePoint On-Premise
This occurs when the web application in which you're working is using Claims-Based Authorization.
The code "i:0#.w|" indicates that the security provider is Windows.
Forms Based Authorization uses "i:0#.f|"
The tag can be stripped using a Regular Expression.