It's a result of a background error check Excel performs for you (this setting can be turned off in the options if you want).

The green triangle indicates that Excel has spotted an error. If you left-click on the cell so as to select it, an exclamation mark icon will appear. If you click on the exclamation mark a small window will appear. The first line in the window tells you the error and the subsequent lines give you options to fix it, get more information about it, etc...

In this case, I'm pretty sure that you have numbers stored as text. That means that they look like numbers (double data type) in the cell, but really they are stored as text (string data type). This might cause problems if you use them for calculations in worksheet functions so it might be a good idea to convert them to numbers.

Attached Images

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