Anyone know what this white stuff is?

Was doing some tidying up of cables and had to remove one of the batteries.

This white stuff was found only along the edge of the inside of the box. It was like small crystals, and when touched it was wet. The weird one was the sludge in the back corner was was running.

Is this leaking acid? In the cold? Something else? What the hell!?!

It's cold and there is a bit of condensation around so a little water here and there.

Yep caused by the battery. Clean it down and see if happens again, if so might be worth getting a new battery.

I assume this is not a sealed battery, but one where H2 gas can escape.
If the crystals are white they have probably come from the negative terminal and are most likely lead sulphate formed by sulfation of the lead plates (and negative terminal if any acid escapes) with sulphuric acid. Pb (s) + H2SO4 (aq) → PbSO4 (aq) + H2 (g) It forms particularly on batteries that are suffering from chronic discharge due to lots of small journeys where the battery does not fully charge. If it is on the outside it will be all over the lead plates inside and once it becomes permenant will affect battery performance.
It is an irritant, mildly toxic and not stuff you would want to put on your cornflakes, so treat it with some caution. Wear gloves, loosen it with a wire brush if neccessary and clean it away with a paper towel. Best way to prevent any more is to make sure your battery stays fully charged as much as possible - invest in a trickle charger perhaps, and next time consider a sealed battery if you don't do much mileage.

thanks guys. I was thinking that was the most likely thing, but the battery is completely clean, nothing at all.

I have had both batteries out, and there is nothing on either and the small drops of white crystals are only on one edge of the box down by the floor. although interestingly thats the side that the negative terminals are on.

If it was leaking from the battery would it not be on the battery it's self? I'm wondering if it is left over crap from an old battery.

If there is nothing on the terminals of the current battery, then either the battery is leaking (in which case get shot pronto), or it is left over from an old battery that was very badly sulfated. I would keep the battery out of the car until it has spent a day sitting on paper towels to check for leaks. If none, then clean the sulfation away and see if it comes back.

will do.

Check the breather hose is fitted to the battery and routed to the outside of the battery box. Most likely cause is gas from the battery collecting inside the box.