It's some sort of electro-chemical reaction
I'm not a genius on this since my only chemical skills involve various alcohols and fruit juices but that's why my tech told me not to run a battery tender here on the island. I'm a mainland transplant and learning little tricks on the island like to take out the restrictive thermostat on the oil cooler since in winter, we see low 70s.
The advice is to seal all the exposed metal with carnuba wax - easy for me to say when I open the can and the wax is almost like vaseline. I remember the same wax looked like a hockey puck in the northwest.
Did you pull the battery and bring it inside the house?