Hi,
I'm sure you'll get various guesses and suggestions soon. In the meantime, does it smell like diesel fuel or transmission fluid? Does it feel oily, slippery? Can you tell if any of your fluid levels are down? Is it showing up anywhere else under the engine, in another bilge compartment, etc?
Based on your pic, and in the absence of the above information, I'm guessing it's diesel fuel, possibly from a loose connection in the fuel line, or from a leak in the fuel tank. If you have access to the front bottom of the tank, rubbing the underside with a paper towel should identify if it's leaking there, as that's a common spot for pinhole leaks.
A bit of good detective work and inspection should reveal what's going on.
Good luck, and let us know what you find.
Frank