how did my oil cap get a milky foam on the oil fill cap?

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Last week changed the oil and filter with mobile 1 synthetic, noticed a milky foam on cap and cleaned the cap. Today sitting at a light I saw a small amount of steam coming from the hood. I found antifreeze leaking but cant find from were. Oil cap has foam on it again. Runs great , no smoke from tail pipe.Oil on dipstick looks fresh. Im stumped. Need other ideas besides a bad head gasket or cracked head on how foam can get on that cap.Please