Is Rhassoul Clay, used in spa body soaps and treatments, a sustainable resource?

In a word, no.

Rhassoul clay is a mineral resource that comes from a natural deposit
of finite extent.  See
http://www.snowdriftfarm.com/form_rhassoul.html. Like any other
mineral deposit it will eventually be "mined out" (or get so expensive
to mine that it is no longer economically feasible to mine).

Rhassoul clay is primarily composed of the mineral montmorillonite,
(Na, Ca)(Al, Mg)6(Si4O10)3(OH)6 * nH2O, which is a member of the
smectite group of clays.  Smectites are "swelling clays" that expand
in volume when they are exposed to water (and, conversely, shrink when
they dry out).  Deposits of montmorillonite-rich clays are often
called "bentonite".

The Rhassoul deposit (in the Atlas Mtns. of Morocco) is but one of the
many deposits of smectitic clays around the world, and we are not in
any danger of running short of this mineral any time soon.

General information on clay minerals: http://pubs.usgs.gov/info/clays/

Information on clay minerals as a commodity:
http://minerals.usgs.gov/minerals/pubs/commodity/clays/

References on bentonite as an industrial commodity:
http://minerals.er.usgs.gov/minerals/pubs/commodity/clays/bentoniteref.pdf

Reference on mineral production (including clay output) of Morocco:
http://minerals.usgs.gov/minerals/pubs/country/africa.html#mo