What is this on my batteries (and second question)

I can't think of any worse place to install a circuit breaker. If it does get corroded, it may not trip in the case of a dead short, or it might trip and not be able to be reset.

One reason I went with catastrophic fuses is that I've never read about anyone installing a circuit breaker on the batteries. A circuit breaker in a more protected location, definitely. However, right by the batteries, the only thing that I see people use are fuses.