Parts store will have some battery cleaner and acid detector. It turns red or some other color when it contacts acid. You might buy a can and spray in there to see if it is a acid spill. Won't hurt to clean it with this stuff anyway it will neutralize any acid that may be in there. If it is acid it could have been an overcharging alternator or maybe a bad battery.
Looks more like mud though. Was there any water ever in there? Are you the only owner? I am thinking could the car been in any kind of flood?