I believe the part you are describing is the bleed screw. You may be able to tighten it down with a screw driver and stop the leak. Typically the bleed screw is used to release air from the system after work has been performed and the coolant replaced.

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2013 X5 xDrive3.5i Premium
2002 530I Sport Package
2001 X5 4.4i
2001 M3