300 SDL mistery leak: What is this?

[julio.garcia.fc]

Hi,

I am very new to my car and still trying to learn. I hope my questions are not too dumb. I am chasing a leak and I think I traced it to this brown cap, which seems to be loose.

What is it?
Is it OK to just tighten it?
I do not think I can get the tab all the way around to tighten it all the way down, which puzzles me.
Should I worry about it?

The leak, if coming from here, is a clear'ish, oily liquid, but it does not smell like diesel. My fluids seem to be OK. Any clues?

Here are a couple of pictures of the location of the cap and the brown cap itself: