A family of Microsoft spreadsheet software with tools for analyzing, charting, and communicating data.
The same happens when you manually (in the interface) set the number format of a range filled with numbers to Text: no green triangles. You can see the numbers, so if you format the cells as text, it presumably is intentional.
You get a green triangle in the reverse situation: you enter a number into a cell that has already been formatted as Text. You may not have been aware of the text format, and if the cells are right-aligned, it is not immediately apparent that you're not entering real numbers, so Excel displays the warning triangle.
Why do you want the green triangle to be displayed?