The notes posted about wearing the SSI of both FUSA and/or TUSA are correct (applied in the ETO). Also the 7th and 9th US Army SSI was sometimes worn by medical units attached to field Armies and/or Corps.
Mind you, there are plenty of illustrations showing medical personnel wearing the Army Service Forces SSI upon arrival in the ETO. And I'm certain that many did not necessarily adapt the insignia to reflect their subsequent assignments or attachments. There was not always time for this, there was no priority to apply the change, the insignia were simply not available...