It looks like one of the succulent euphorbias. It may take a while to find the right one--there are hundreds! :)

I'm thinking a young E. cereiformis, at the moment.

I think you are right.

Oh. I thought it was an Astrophytum but don't know much at all about euphorbias...

The true cacti, such as Astrophytum, have sunken, more or less wooly dimples on the ribs which contain the lateral growth buds, flower buds, and 1-20 spines, if any.
The cactus-like Euphorbias have waxy or resinous lateral buds in the same positions, flanked by two short spines. Most species also produce small, short-lived leaves just below the buds when actively growing. In some, the the short stems that carry the tiny yellow flowers turn into hard, sharp thorns when the flowers fade. E. cereiformis is one of the most cactus-like species. The flanking spines are tiny, the leaves are all but invisible, and the "flower thorns" are numerous and tough.

Tank you Tug. I did notice that it didn't have spines. Never actually had an Astrophytum though it was always n my wish list...

My personal favorite genus, myself!