Corroded metal plate under driver's seat

Just bought a cherry 2012 Sentra Special Edition for daughter.  When the driver's seat is all the way forward, noticed a metal plate (black) covering wiring harness spinoff going to something underneath that plate.  Problem is that cover plate was fairly badly corroded with a white corrosion.  The car only has 18K on it, and everything else about the car was excellent.

Bought a small can of semi-gloss black and repainted just the visible (top) part of the plate.  The thing underneath it is still whitish with corrosion or something.  Looked almost like someone deposited a bunch of salt from their shoes while sitting in the back eat behind the driver.

In any case, I didn't deem it to be a problem necessarily, and after painting the top of that piece of metal it looked nice and black again all over.

Question is:  What is that beneath that protective plate that has harness wires going to it?  Just want to know for my own sake.  Is that the exposed top of the catalytic converter with the oxygen sensor wires attached to it or what?

Would love to get an answer just so I know.

Thanks much!!