Build up due to moisture. Just drive the car hard every now and then and it should be fine. My father's 330 had similar gunk developing under the cap attributed to the short leisurely commute that wouldn't even fully warm up the temp.
'02 ///M5 Imola Red / Imola Red '09 335i Alpine White / Coral Red
'04 E55 AMG Brilliant Silver / Anthracite
'96 318i Alpine White / Black
'01 330Ci Titanium Silver / Coral Red
'96 328i Jet Black / Black