Jane, is it possible this plant got a little too cold or have you been watering it a lot? This appears to be not corking but the beginnings of mush and death, I'm sorry to say. Astrophytum are known to be a bit finicky, particularly during winter.

Thanks, I have had another one get the mush and die and it happened fast , thats what confuses me, it hasnt gone below 40F and not overwatered and this has been happening for about 1 to 2 years and still firm Jane

Then I'm happy to be proven wrong. Corking, btw, is generally brown in colour and looks like flakey dehydrated cactus skin (because that's what it is). I don't know what your plant's discolouration is, but if it's not soft, that's a very good thing.

Thanks so much, yes I have corking on many of my cactus, it gets so hot here. I'll keep an eye on it Thanks for your help jane