Numerous cells
have a small green triangle in the top left of the cell.
This may be a separate problem. The green triangle indicates a
potential formatting error. If you select one of these, you should
have a little menu suddenly appear right next to it. Go into that and
it will tell you what Excel thinks is wrong, usually a number that is
formatted as text or other similar issue. Some of these are just
cosmetic, others can affect calculation.

You can pick which errors Excel will highlight with the green triangle
in Options on the Error Checking tab.