Is this a clam?

[Tesk]

I've had this frogspawn for few weeks arrived pretty beat up with 2 dead heads, black sponge on it as well, I did get some money back

Just noticed the side of it open and close a little.

What is that?!

 

[dedragon]

yep some type of rock boring bivalve, I have had similar looking ones and they were harmless