This really doesn't look like battery acid but if it is , it will have a distinct smell . It looks more like grime from something in that area leaking causing dirt , etc. to build up a film ! At 12,000 miles , I wouldn't think it would look that grimy unless it's been subjected to a very dusty or dirty environment ? If this is a FWD car , the AC compressor is usually in this area ( on passenger side of the engine compartment ) and leaks in that system often lead to this type of build up . Or it could be your power steering pump ( also normally in this area ) leaking ? Is the battery the only component showing this grime or is it also built up in nearby areas ? Did you buy the vehicle new or used ? Did you buy the car lately or have you had it for a while ? Did you get a vehicle history when you bought it ? Have you looked under the car for unusual debris or signs of water damage ? Raise up the trunk liner and look in the hard to reach areas ( such as the spare tire well ) for signs of unusual residue . More info would be helpful particularly make and model of vehicle . Good luck !