Would only copper be harmful?
It looks more like flecks of iron pyrite. Copper does not exist in elemental form naturally, it's tied up usually an as oxide or sulfide of some sort. Iron pyrite is ferrous sulfide which is typically insoluble in water but can be oxidized to iron oxides and sulfate compounds. Iron compounds are naturally acidic in nature so it may impact tank pH.
Is this from an existing rock in the tank or one you plan to add?