The ocean is 20x supersaturated with calcium and is constantly being replenished by geological stores of the stuff coming in through rivers. As long as the ecosystem remains supersaturated this shouldn't be a problem for any marine life. Though a slight dislcaimer there is i'm not a marine scientist!

I think the diffusion rate of Ca and CO2 is also important. Even though the ocean is supersaturated *globally*, the local area around your plant might not have enough concentration. Hope you can get someone to check that.

Might have to spread your plants across multiple regions, or place it around somewhere with low impact to marine life I think.

They can reduce that risk by having very long tubes that can be re-positioned from the surface using ships to take water from different locations while the currents replenish the other locations. Or just have several tubes to draw from. Like rotating a pasture. No doubt they will need to actually monitor and plan for this or they will likely create a disaster.