As Mike said, it will be a very long time before anyone is mining an area 100 miles wide, if ever. Now, let's suppose that some day in the millenia to come, that actually happened, for reasons that will be intelligible only to the people of the future.
Could someone looking at the moon from Earth see it? Well, 100 miles subtends at arc of 1.4 arcminutes at a distance of 238,000 miles, and the naked eye can distinguish 1 arcminute, if there is 100% contrast between the foreground and the background. Which means you might possibly be able to distinguish a dot on the moon, if you have perfect eyesight and knew where to look, and if for some reason they painted the surrounding terrain white and the mine solid black, or vice versa.
There's the rub. If you dig up lunar regolith, what you see beneath it is--you guessed it!--more regolith! The best spots for surface mining are the dark maria, and what you find under the surface is more dark maria. Basaltic soil, mostly.
So my guess is that even if someone were to excavate the surface in a big square 100 miles on a side (that would be more than 6 million acres), you would have a rough time finding it even with a very good telescope under perfecting seeing conditions.
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Who are all these people and what do they know? 
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