An Internal Worker is a stereotype of a class that is specified in UML Extensions for Business Modeling. It can be shown as a regular class rectangle with stereotype of "internal worker", or as this icon:

An Internal Worker is a Worker that interacts with other workers and entities inside the system.

To draw an "internal worker" on a Use Case diagram, select the Worker icon from the tools menu, then opening the class definition and changing the stereotype from Worker to "internal worker".

See also Changing from icon view to class view.