Circuit breakers with battery acid look

So, what causes the battery acid look on circuit breaters? I think i remember thsi but don’t recall it. Let me know your thoughts. 1900 house with near breaker panel. Some old sheathed knob and tube but mostly romex.

Moe 044 (Small).jpg

Moe 045 (Small).jpg

It appears to be moisture problems on the breakers, but the panel itself doesn’t seem to show any signs. Perhaps in another picture it does.

That is the thing. It was very clean. Some stuff on the breaker screws and a few on the main breaker.