It's usually caused by a leaky battery and pretty hard to clean off (if you're not a science buff).
There are two very handy and basic household products that can take care of that for you. Baking Soda and Vinegar.
I tried vinegar. Wow. I love science, don't understand it but love it.
In my case the battery plug (for want of a better word) was caked in corrosion, within seconds of soaking it in vinegar the corrosion was nonexistent!
I'm pretty chuffed with the result.
