The lather/soap-theory doesn't convince me: soap and lather dry far to quickly to penetrate any significant depth into the stone. And as it is very soluble in water, the effect wouldn't be very long lasting - especially after repeated lapping, rinsing and soaking. Also: detergent would have bubble-generating properties with and without slurry.

Especially after seeing these latest pics, I think it's oil residue. It is a largish stone, as used by carpenters. They often used coticules, especially the harder ones, with oil. This changes the colour to a particular type of bland hue with light (even greyish) and dark blemishes, especially when dry, at the same time lending it a more 'transparent' (butterscotch texture like) aspect.

Oil would eventually penetrate a coticule, and can't easily be gotten out. A mixture of oil residue and very fine (because hard) coticule powder could produce a bubbling mixture with water. It would explain the difference with and without slurry.

I've got a large carpenters' coticule that was used with oil too. I don't usually use it with slurry, but will try and see if I can get to make it to bubble.