Green Mushroom's and clam

[whitebird]

i got this about 2 months + ago and its 2x3" + in size the lfs said it came in a order they got not knowing it was a clam for about a week or so and is this a common thing thats seen? and does any one know what kind it is and how long do they or might live in a tank ? here is a close up and in the other its to the left of the Colt

 

[whitebird]

this must be a pretty common thing to see ?

 

[jackri]

I've never seen one. Looks cool though. I'd be interested to know what it is -- besides a bi-valve from the looks of it.