The Document Inspector found hidden Excel Survey questions in your workbook. These were entered in Excel Online and saved with the workbook, but they aren't visible there.
The Document Inspector can't remove these survey questions for you because it can't access them in Excel 2013.
You need to open the workbook in Excel Online to view or remove the survey questions.
With the workbook open in Excel Online, click Edit Workbook > Edit in Excel Online.
Click Home > Survey.
To view the survey, click View Survey.
To remove the survey, click Delete Survey.
To return to the Excel desktop application, click Open in Excel.
If you deleted the survey, run the Document Inspector again to verify the issue is resolved.
More about surveys
For more information about Excel Surveys, see Surveys in Excel, hosted online.