the answer is that any copper in the rock will probably leach into the water column as copper in the water column goes below the copper concentration that originally got the copper in the rock.
With active plant life growth and the harvesting of that plant life, the copper in the rock will be exported from the system.
If you have algae growing on that rock and snails eating the algae from the rock you should get a good indication of how dangerous the rocks is. If the snails are healthy and growing, there is probably no dangerous levels of copper in the algae they are eating. And therefore no dangerous levels of copper in the rock.