Hatiora salicornioides in nature 

The ones shown did grow on just rocks on open areas in the woods

They are very tolerant to different conditions. They may look different depending on how you cultivate them. In a more moist environment they will get greener and more grow. However they will stand periods of dry as well. In winter at low temperatures they can be kept dry for months

I threat them as real cacti. This one can have lots of sun