Joe,
Beautiful plant, great spotting. I had an Astrophytum myriostigma v. quadricostatum fa. obesum from seed that was variegated, well two actually, just like yours. They contiued to become more yellow as they got older until last summer, one of them died due to lack of chlorophyll, it happened so fast that I couldn't get it grafted (no growing stock available at the time. Hope yours doesn't do that.
Dan
BTW, did you start this from seed?
Some of my variegated Astros, the first two are the same plant getting more yellow, the other three are of another plant that finally died. Yes those are pups forming on the top.

Here is a link that might be useful: My Variegateds

Yes it is from seed and on its own roots.