Just ignore the green triangle. Even better, go to File > Options > Formula, deselect "Enable background error checking" (click to remove checkmark), and click Reset Ignored Errors to remove any other green triangles.
First, Excel calls these "errors", but they really are warnings.
Second, as you noted, often they are not mistakes at all, but something that you choose to do intentionally, and Excel mistakenly thinks it is wrong or deserves "some thought".
If you select the cell and hover the cursor over the "!" symbol next to the green triangle, it might tell you that formula refers to an "incomplete range"; that is, a range of cells that has additional contiguous cells nearby. For example, you might have values in A1:A100, and you write =SUM(A1:A10) in B1 in order to sum only the first ten cells. Of course, you made an intentional choice. But Excel thinks you might have intended to include the larger range.
Arguably, occassionally, Excel's "error" checking does highlight a legitmate mistake. But in my experience, that is very rare. More often, the "error" checking is just a distraction. It causes you to have doubts, even though you are sure that everything is okay.