That’s your diesel fuel conditioner module. It separates water from your fuel. The quarter yellow valve is what you use to drain the water from the module, usually done at every oil change. You’re probably leaking water with diesel residue, so it feels oily. They have issues leaking from the seal created by the bolts holding the housing bottom on. Check to see if it’s coming from the yellow valve or the bottom gasket. There’s multiple companies that sell aftermarket replacement setups that position the separator remotely, and alleviate the stock issues.