I started seeing a whole bunch of tiny white spiral shaped snails yesterday.  "Cute!" I thought they were baby Cerith snails.  Then today, I noticed that my Tridacna Clam wasn't as happy or active as usual and noticed about 15-20 snails climbing all over it.  Hm, I better check the Internet, I told myself.  I searched the archives and found out that these snails are parasitic.  
This link has some info on the Pyramidellidae snails in my tank.  
 
http://www.exotictropicals.com/encyc...9/fnc0999.html 
So I took the clam out and brushed the snails off with a toothbrush.  I don't know if the clam is going to make it though.  The clam, though still alive, looks weak and there probably are more snails in the tank.   
I read that blue legged hermit crabs will eat them.  If so, its time for me to buy about 25 or so of them.