Not just coral will be effected by Nickel, but other important invertebrates as well such as amphipods which play an enormous role in the food chain and health of reef environments.
And we’re talking parts per billion (ppb) when heavy metals can start to become toxic.
And correct me if I’m wrong, but Manganese is another element often found in olivine, which is another metal that can be toxic to marine life.
Nickel is a more reasonable concern; nickel sulfate's lethal dose is on the order of 250 mg/kg for humans, so around 10 or 15 grams for an adult. It's not really in the same category as things like mercury, lead, thallium, barium, or even cadmium.
Invertebrates are much more sensitive to heavy metals. Again toxic doses for many marine invertebrates are in the ppb-ppm range.
Copper for instance is commonly used as a treatment for marine parasites on fish. However the dosing needs to be done carefully, since a doses in the range of 5ppm can start to kill the fish themselves.