Some succulent plants refuse to budge and are very slow growing. This Senecio is one of them and it is probably the reason it is rarely offered for sale except as a collector's plant.
A common name given to it of 'Cocoon Plant' refers to the resemblance the leaves have to silk covered insect pupae cases. This 'silk' has a traditional African use of being collected to use as tinder and it does peel off the leaves revealing a pale green underside (just visible in this photo).
Silver foliage plants can be tricky to keep going during periods of high rainfall or humidity so I have incorporated lots of grit and perlite into the potting mix of this plant to make sure it is free draining.