And then it will settle on the seafloor again. How did it end up there in the first place? By being denser than water.
Absolutely nothing to be concerned about.
Concern for the planet and climate is a worthy cause and ought not to be discredited by obvious nonsense like this. Extreme reactionary climate conservatism is ultimately self-defeating, much like it's less sexy social conservatism cousin.
Sea floor mining will happen at 200m - 6500m deep. The recently granted exploration license for the Clarion-Clipperton zone is at 4000-5500m deep. There's very little life that deep, and certainly no sunlight to get absorbed by murkier water.
Still, it's nice to just dismiss it as hyperbole instead of thinking we need to take serious action to prevent further damage to our planet.
You can say this about every climate disaster.