[EC900]

Agree that the substance can be something other than battery acid. Instances of a leaky washer pump/line could be the issue.
My first approach would be find out what the stuff is.
There’s battery post cleaners that will change color indicate battery acid. I use Noco Battery Cleaner and Acid Detector regularly to clean and keep clean posts, even sealed batteries have to vent corrosives and off-gassing over time.

Put a straight edge along the sides and bottom of the battery, any bulging with indicate an overcharging issue.
There’s also possibility the battery case had a small crack when you bought it, maybe it was dropped on the corner at the store?
Although there are many brands of batteries all made by but a few manufacturers and rebranded, I stay clear off such off brand store labels.
For what it’s worth, I’ve found unused AA Duracell batteries leak in the package, so every now and then malfunctions are going to happen.