⌛ help ID rock growth

Hello

I have just returned from Cuba & would like some help identifying something I saw on the shore there.

There were many small black pellets in the area.
There was also a few small to medium size, thin clam like shells attached to nearby pieces of rock.
There were what appeared to be lobster tails growing out of or upon rocks, both dry rocks near the water & submerged rocks. There were 'lobster tails' that were black & hard, as well as 'lobster tails' that appeared to be less mature and were gray & less hard.

I did not try very hard to dislodge one of these 'lobster tail ' things; that would be against my belief of leaving things alone, but I was most curious - certainly a gently poke & tug did absolutely nothing.
My best guess as to what is happening is that the little clam like things attach themselves to rock, emerge in their protection & form a new outer shell that looks for all the world like a lobster tail.
The little black pellet things seem to congregate around the formation.

Any input ??

Thanks !!!

 

The segmented animal on the right looks like a chiton. The small black things look like mussels.

 

THANK YOU so much !!!
I've been searching the internet but could not find ANY hints !