Hello Angel, I saw your project and I was thinking about the possible consequence of the use of grinds mixed with concrete in salt water.
Concrete has, at best ipoteses, a durability of 10/100 years depending on the mixture (chemical) used. When in contact with salty water, the durability should come down steeply. - https://www.tandfonline.com/doi/pdf/10.1080/19373260.2010.521048
However rubber and other plastics may take 20/1000 years to decompose. *one reminder, most of the rubber used in the manufacture of soles is not natural but rather synthetic, so it uses above average chemicals
In the short term, I believe it works and is feasible once the corals disappear with each passing day, but in the long term I foresee a series of particles and pieces of these grinding beings dragged by the current serving as food for marine life.
Do not be discouraged, test, have proof that I'm wrong. I really want to follow your project. Good luck.