The coil is very exposed to weather, air and moist. Better cover the coil a foil. Clean it first with metal cleaner then cover it.

How old is the AC?: if the oxide is not too much just remove it with cleaner, if pipes are rottening, then you will need to replace lines.

I doubt that it is bad copper. It could be outgassing from the crematorium. you can get a coil cleaner spray, but that doesn't explain what caused it in the first place.