You can put aluminum foil into your salt water tank. Just as long as you are trying to create a DIY battery. If not, bad plan.
Shading GSP from light is not an effective solution, as you will find out after you restore the rock to lighting. It will grow right back. Your best option is to remove the rocks and trade the rocks for new rock that doesn't have GSP. You can try to scrub it off, but you have to be very thorough or it will grow back. You can kill it if you are willing to kill the entire piece of rock, by setting it out to dry in the sun or soaking in fresh water for a couple days. Both will create a stinking mess, but will allow you to restore the rock after it is dead and cleaned.
S !