Yeah, it looks like a pale form. The white one has leafy stems.

That's what I think too, although I don't know what you mean by leafy stems.

There are two obvious color differences, the flower (the "slipper") is missing pigment as well as it's surrounding leaves. Also the lower leaves are slightly stunted looking and lack ridges. Both of the white examples I found looked this way.

He means that the white one (Cypripedium arietinum ) has leaves on the stems of the flowers, not just at the base of the plant. I looked up that one too.

Here is a link that might be useful:

Pink lady slipper likes acid soil, so acidity shouldn't have any affect on the color. The cause is likely genetic. Most orchids have had various forms named, the most common is forma 'alba' for white flowered forms. Such forms are just genetic anomalies occasionally found in orchid populations and aren't usually considered taxonomically significant.

The white species I was thinking of is Cypripedium candidum, but most lady's slippers have leaves on the stem. The second part of the name of pink lady's slipper "acaule" means stemless.

Oh that one - it wasn't listed for Maine so I didn't check that one when looking at white ones.

Thanks for explaining "acaule" - I didn't realize that.