Q: Mechanic issue

Hello,i recently took my car in for a full major engine service and left it overnight ( coolant,oil,fuel filter,oil filter,air filter,ac re-charge,spark plugs,brake fluid,power steering fluid) i went out today and there was a thin layer of white sludge on the oil cap which was NOT there before i took it in for the service. However,when i went to collect the car i opened the bonnet and the mechanic allowed me to check the oil before i left and before i paid for the service and i lifted the oil cap an the sludge was on the cap. Is this a headgasket leak or something worse? The coolant/antifreeze is clean and all fluids have been changed with top quality fluid