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.
Imola Red / Imola Red ///M5
E60 SSK, Dice iPod, Illuminated ZHP knob, GT Custom Exhaust, BCR Coilovers
'01 330Ci Titanium Silver on Coral Red
'96 328i Jet Black on Black