Why is there foam in the oil cap?

WHAT COOLER IS THE FOAM?

Condensation is fairly common on the oil filter cap in colder weather on a lot of engines. It's most likely just water condensed from the normal blow-by gasses. If your really concerned monitor the coolant level for loss, when you change it in a week have an oil analysis sample bottle on hand to send it to a lab so they can test for coolant.