Looks like its the galley plug in the picture.
Wash it down with degreaser and dry it thoroughly then remove the plug. Odds on it will have a washer behind it that needs replaceing. BMW have used alloy washwers in the past which corrode and then leak. Fit a brand new copper one and you'll be fine.
If I were you I'd order a 10mm brake washer - they're cheaper than a postage stamp. Can't be sure without looking but its probably the right size. if not then clean, measure, reassemble with the duff washer (you'll be ok for a few miles) and then buy another of the right size.
Do clean it up before removing the plug though, any of that grit getting inside your engine will ruin it in double quick time.
Mark