If you can get the car started (jumpstart etc) and have access to a voltmeter pop it on the battery and check the charging voltage with the engine idling. Should be no more than around 14.2 volts. With what you are describing (wet on top of te battery) it sounds a lot like the voltage regulator is not working correctly causing an overcharge condition and the electrolyte is boiling and venting into the tray.

Failing that have a good look at the battery case and see if there is a small crack that might be leaking battery acid (electrolyte) into the tray. If all that is OK, it may be as simple as someone may have just simply overfilled the battery and it is leaking out causing the mess. DON'T get any of the build up on your skin or clothes.... Just wash it away with plenty of water and give it a clean-up with a mild soap and water solution and an old brush that you can throw away afterwards, dry the bits and then a quick hit of etch and matt black - back in business!