Can anyone tell me what the filler cap is that is just to the right of the battery (driver's side) with the green cap?

After I got the last service onmy car I noticed fluid sweating out onto the plastic cover. The cap is so tight I can't get it off by hand and I don't want to force it.

Any idea why it would leak?

Thanks,
Bill.

 

If it isn't brake fluid or washer fluid, then maybe power steering fluid?