This will be from an electrochemical reaction between the can (mostly iron I think) and the water and possibly the aluminum (a better chemist than me would be able to tell you).
Aluminum has a fairly impermeable mono-molecular oxide layer on its surface (incidentally this is why you can color coke cans and the like; the color is in the oxide layer). The oxide layer in the case of your pot has probably been compromised by the reaction between the can and water, possibly scavenging some of the oxygen from this layer.
To remove this you can do either of some scrubbing with a steel based scrubber - basically polish the pot surface, or you might be able to remove it by boiling the pan with a weak acid solution; vinegar is probably the easiest, but any acid should do. Note that both will make the pot shiny, and you should do a short boil with some water to re-establish the oxide layer.