mustang II rack & pinion question

[kpdalton]

So I have a small leak on my mustang II rack & pinion and it seems to be from the large (I think plastic) nut more towards the drivers side section facing fwd and down. I tightened it down some and it easily turned. But I assumed this was just a cap for some adjustment inside. I don't think its fluid running down to this area but there is that possibility. I inspected it very well I think.

So is this a nut that should be tightened to avoid leaks or just a cap?