Hello RoseSea,
In 30+ years of working in medical chemistry, I have never heard of rosalic
acid, and a quick search of several chemical catalogs didn't turn up the name.
There is a rosolic acid, which is a colored compound sometimes used as a pH
indicator, but I cannot imagine any reason to use it on shells.  In any case,
the last four letters of this chemical name, a-c-i-d, are really all you need
to know.  Acids dissolve carbonates, by transforming them into water and
gaseous carbon dioxide.  Mollusk shells are calcium carbonate, and acid of any
kind will dissolve them.  Weak acids will dissolve them slowly; strong acids
will dissolve them rapidly.  There are times when the judicious use of a
little weak acid (such as vinegar) may remove a stain from a non-glossy shell
surface.  But even then, we should realize that the way acid removes such a
stain is by dissolving the layer of shell on which the stain was located.
Removal of heavy encrustation like barnacles or coral with acid is usually
futile because a strong acid is needed to dissolve such encrustations, and a
such an acid will dissolve the shell just as fast as the encrusting deposits.
Paul M.