Gray green substance under oil cap - Cummins ISX15

[SupVit]

Hello.
2015 Prostar with Cummins ISX15 2350. 370k miles. I'm the owner since 197k.
I decided to change the oil, warmed up the engine to 140F, opened the cap and found a dark gray-green oily substance. On the cap and about 2 inches into the yellow filler pipe. Also, air comes out little bit from the filler pipe. It can be seen under the light of a flashlight, like in one video on youtube channel where Rawze checked for the green Prostar. I don't know how much air should come out, I have never opened the cap of a warm engine.
I opened the breather filter too (I need to change it, although it does not look dirty, I changed it 130k ago). The oil on the filter is not dirty. The plastic under the filter has small amount of oil in which there are beige drops. Around the cover there is also a gray-green substance. The cap of the coolant is clean. Level does not change. For 2 years of owning a truck, I have never added any coolant or oil between replacements (12-17k).
What is it? Looks like coolant in oil. But substance is dark gray instead of milky. Where should I start looking for a problem? I think the problem appeared recently, because 13k ago I changed the oil and the cap was clean.
Thank you.