The surface of the earth has the benefit of being weathered, with most of the nasty stuff leached away or covered up, but it can be surprising much stuff the water running through debris or mine holes can pick up. Especially since we tend to choose to make mines in areas where there’s especially a lot of nasty stuff.
I can’t speak to the actual danger presented, but this is a pretty reasonable thing to throw up a fuss about.
It is debatable whether stopping a volcano is harder than stopping a mining company from ignoring it’s responsibilities.
Even then, it can sometimes result in increased pollution of drinking water sources, in which case remediation needs to happen. Given that most landslides tend to happen in poorer countries, this isn’t always the case.
Mining waste includes the chemicals from the ground, and chemicals used in the mining process (and chemicals produced in the interaction between the two.) Obviously, with significantly less of whatever was being mined, since the whole goal is to extract that from the first by use of the second.
tbh I don’t see a problem with dumping rocks and dirt in the ocean