ID on something new growing

[mattybecks]

Hello guys.
I haven't collected anything for about two weeks now, but I suspect this has been growing for about a month now. I only noticed it yesterday though.
Its hard. So I'm guessing whatever it is involves calcium.

 

[Dloks]

It’s a clam of some sort I believe lol

 

[tlindsey]

It’s a clam of some sort I believe lol

+1

 

[mattybecks]

I found a second one growing too when I rearranged some of rocks this weekend.