You can buy acid at the drugstore - ask you pharmacist.  Or try a
swimming pool chemical company.
 
I have a large, beautifully colored Lion's Paw (_Lyropecten nodosus_)
that has some coral growth on the nodes.  Does anyone know what it would
do to the shell if I tried to grind it off with a Dremel tool?  Would the
shell be damaged underneath?  It is in perfect condition except for that.
 
Sylvia Edwards