hmm... your description makes me think the electrolyte escaped - that is potassium hydroxide in most batteries (according to google. )and is even more base than saltwater.. thus the reaction (maybe... gary could probably do a far better job w/ this than me, or for that matter any of the teachers we have here).

other than the steel case, everything else seems fairly inert, and far less deadly than elemental copper...

I think what probably happened is your PH * ALK skyrocketed to unholy and ungodly proportions... and zinc, potassium... all these things feed plants - thus your algae problem. light that sump up and throw some cheato in there.

Positive cap: Formed protrusion at one end of the battery can which identifies it as the positive terminal.
Steel can: Nickel-plated steel which is formed into a container to hold chemicals and serves as the positive collector.
Outer Jacket: A plastic sleeve which contains decorative printing identifying the cell type and size.
Separator: Porous non-woven fibrous material which separates electrodes; holds electrolyte between electrodes.
Electrolyte: A solution of potassium hydroxide in water which carries the ionic current inside the battery.
Cathode: Manganese dioxide and graphite which take up electrons from the external circuits.
Anode: Powdered zinc metal which serves as the source of electrons.
Anode Collector: Tin-plated brass which serves as a path for the electrons from the anode to the external circuit.
Seal/Vent: Molded plastic disc which holds internal components inside the cell and releases internal pressure when battery is abused.