Do you see this in the "Worker Layout" (or "Farm Layout", depending on your QubeGUI version)?
Do you (or did you ever) have a worker running on your Qube supervisor? Check the "lastupdate" timestamp for the supervisor entry in the "Properties" pane when you select the supervisor's entry in the worker list.
It may be that this worker entry is old and is not updating because the worker service is no longer running on the supervisor host. If that's the case, you should refer to section 3.11.3 "Worker removal" in the Qube Administrator's Guide.