When Excel open it is constantly checking in the background for potential errors in your worksheets.
If an Excel detects an error (or at the least what Excel thinks is an error) then the cell is “flagged” with a small green triangle in the upper-left corner of the cell.
If you don’t want Excel to check for errors you can turn the feature off by following these steps:
Click File > Options
This will display the Excel Options dialog box.
Click Formulas at the left side of the dialog box.
Clear the Enable Background Error Checking check box.
Any existing green triangles should disappear and Excel will stop checking for errors.
If you would like Excel to check for some errors and not others you can modify this by disabling some of the options in the Error Checking Rules section.
If you are using some of the Error Checking Rules you must leave the Enable Background Error Checking box checked.