Mom came In to eat her lunch and her face had a grey look to it. What could that indicate?

I found with my mom in her 90s that she could have good days and bad days (or even weeks). And there was never any real correlation with anything that I could figure out. I wouldn't freak out if your mom isn't showing any other symptoms, just keep a close eye on her to see if anything else starts to happen.

Yes I will contact her doctor Monday. She looks so tired. I've had her in recently and she was doing fine then. Thank you

We aren't doctors here (most of us) and not seeing your mom in front of us, it's difficult to know what to tell you.
Dehydration, lack of oxygen, low bp all spring to mind. But talking to her doctor or taking her to urgent care will get you an actual answer and not a guess.

Can you do that?