Super Sculpey mixed with Super Sculpey-Firm. Not too sure on the ratio, maybe a bit more Sculpey-Firm than plain Sculpey which is why it's more grey than translucent. In previous sculpts, I've added Fimo black to make the overall clay darker.
The other thing with these sculpts is that I've tried to keep them warm so that it's easier/softer to work with over a duration of time. In the past before I've found leaving sculpey for a while can sometimes dry out, making it seem like it's been baked (but not). If you try to work it, things start to become crumbly. So with these sculpts, even before applying the sculpt tool, I warm it up a bit by rubbing it on my fingers and not being afraid to hold the actual sculpt betwen my fingers to help keep things warm. Rubbing the tool also checks if it's smooth or if it's had any dings since you've used it last or if somethings stuck to it.
I also have a blob of clay next to the sculpts which I draw upon if I need to add bits. Every so often I'll remix that blob to bring it back to life. Again it's just making things more malleable, softer, and adding warmth before applying to the sculpture. We're talking body warmth here, not under a lamp or other heating device.
Hope it helps!