[hackily]

I'd be fairly concerned if undersea mining waste or debris started killing off phytoplankton, perhaps by disrupting any of the delicate cycles such as phosphorus, or providing extra iron which may lead to runaway algal blooms or such, and subsequent algal death and oxygen shortage. Since the mining is so deep, we don't know if these effects will stay localized, or if undersea currents will dilute these effects, or anything.

It should be valid to question what harm there could be. Nature is quite fragile, and we should know that, especially seeing what harm industrial processes have had on the atmosphere.

On the other hand, it's quite possible that undersea mining may produce less harm than surface mining, where waste gets dliuted far enough to have less impact. Maybe similar to how salmon farming has to be done where there's a strong enough current to dilute the waste.