Battery acid is sulfuric acid. It dissolves a lot of things. A spark is not guaranteed to happen with exposed plates. But with enough tries the odds go up.
That looks like a cheap wall wart. I doubt there is any circuitry in there to switch to a trickle charge when the battery is fully charged. It should be a battery maintainer that can charge the battery and then switch to trickle mode or stop charging altogether until the battery has discharged about 10% or so and then recharge it.
The electrolyte didn't actually boil. When charging continues on a fully charged battery the water molecules are separated into hydrogen and oxygen gas. That is what bubbles up through the electrolyte to create the mixture of hydrogen and oxygen gasses above the electrolyte. That mixture is very explosive.
cheap walmart? NO that photo's bottom shows it is a factory generac maintainer/charger, personally i have had 3 fail in my unit over the last 9 years, 2 would not charge,each had a burn mark in the plastic casing where something inside overheated from power surges, 1 overcharged like the Op's .overcharged batteries overheat make major explosive gasses where any spark makes them blow up, overheating melts the plastic like he posted a shot of , proper dress and handling of any battery is a must do, esp goggles over one's eyes and long sleeve shirts .even for this 70 plus year old retired master auto tech who has had many blow up at him trying to jump start a engine,
TG the op did not suffer a major injury or get blinded