battery acid

Greetings! Happy Holidays/New Year in advance! I have a couple of Yamaha basses, they are active and the situation is that the battery box is kind of different from the generic enclosures, the situation I'm having at this moment is that now they looks like rusty or some damage due to the...

I’ve been been using Duracell 9 volt in my bass. But recently here for the last couple of times when I would check my battery, there was this oily substance on the battery? Any ideas on what could be causing this and is it a major concern?

There's a bass in the back of my closet that I haven't touched in about 2 years... The scary part: It has 2 9v batteries and I have not changed them during all that time :nailbiting: I'm afraid to open it. What horrors lie within?