Can't figure out which is the hitchiker?!

Yesterday we bought our 2nd coral at our local LFS. It's a small colony of green striped mushrooms. We were thrilled to notice a hitchiking white/green fan worm too. We got them home, placed them at the bottom of the tank and OMG the whole rock clamped! We aren't sure what kind of crustacean it is but it seems to be melded into the rock and opens only about 1/4 inch. So is the rock the hitchiker or the shroom and fan worm? What are our corals attached to? A clam? A muscle? An oyster? The rock is about 5 1/2 inches and the un-named critter is about 4 inches of that!
Here's a pic check out the outline of the critter. It's the orange arc but it goes all around. I haven't figured out how to put multiple pics here but they are in my gallery.

Sheeesh!

 

The hitchiker is the coral. you scored with that purchase. may be an oyster or clam. you can post your picts by going to the photo gallery and clicking on the pict. once the larger pic comes up click on the BB code line and click copy. go to your article and click on edit. double space below current pic and paste the new pick. click save changes and it should come up as part of the response below the 1st pic. do same thing for each add pic you want in the same post. congrats on getting a freebee. now all you need to do is decide how to feed it to keep it alive.

 

Any suggestions? I know it's some kind of plankton but does phytoplankton sound right? Do I turn off the pump for a few minutes and use an eyedropper and drip some near the crevice?

Can you help me figure out what the heck kind of bivalve it is? It obviously doesn't require lots of light but I'd like to know how to care for the thing! We named it Igor LOL

 

would need a clear picture of it from the front slit when it opens., anyway :

Scallops- Substantial amounts of phytoplankton are neccessary for good health and good growth.

Oysters - Same as Scallops

Barnacles - Can be fed enrighed artemia or some other moderately large high energy foods. Phytoplankton are too small.

Clams - same as scallops

I have not been able to keep any of the above alive (except clams) for more than about 6 months. assume due to starvation, just could not get enough food inum without polluting the system. good luck.