This problem occurs if you are constantly doing short journeys! i,e. the oil does not get hot enough to work properly! therefore you get this mayo problem under the cap!
another problem is that you have somehow got condensation or moisiture in there, and the oil has reacted with it!
I would advise taking the car for a very long drive to see if it clears it!
Or another advisable would be to drain all oil, stick new filter in aswell, and clean as much mayo off the top as you can!
Jake