Specifically, copper kills algae and silver kills bacteria and viruses. The Romans used copper pots to store water for that reason. The settlers coming to the West would toss a silver dollar into their water barrels on the wagon trains. NASA used a silver cell generator on the Apollo space missions to purify the spacecraft drinking water. Copper and silver ions, when distributed throughout your pool in minute quantities, kill algae, bacteria and viruses.
Ionization is completely safe.
Most ionization systems operate at 5 times below the EPA drinking water standards for copper and silver.