What's this oil leak? What am I looking at?

[Boleador]

Thanks in advance for the help, and my apologies for my ignorance!

I was replacing the brake wear sensors on the rear passenger side, when I noticed this oil leak. There's not enough of a leak for it to drip onto the floor, but oil is clearly leaking out somewhere. Can you guys help me to identify what part of the motor this is? What is this hex head/cap and this gasketed interface that appears to be leaking? Is this common?

Thanks!