The colour profile of the document gives it a green tint. Assign (not convert) sRGB or Adobe RGB to it and it becomes grayscale.
You can also set an adjustment layer to Hue/Sat and run the saturation to -100 after conversion. The way I do conversion results many times in color "leaks" and that's how I handle them.
In Photoshop, of course. This doesn't work in ACR.
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The Black & White adjustment in the document is already producing pixels where R = G = B.
A Hue/Sat adjustment cannot make the RGB values any grayer than gray.
It's the document's "SFprofT" profile which is causing a greenish appearance when displayed.
Thanks to everybody!
Ok, I see that.
Then the other option is simply to convert from RGB to grayscale. Image>Mode>Grayscale.