When you get milky engine oil on your engine oil cap it's usually caused by condensation and moisture in the engine due to driving short distances or not getting the engine up to a good enough temperature to evaporate the condensation.
Condensation naturally collects in an engine through the combustion process and the breather and the moisture in the engine will do no harm to your engine as long as it's just moisture.
However sometimes coolant can mix with the engine oil due to a head gasket leak or an intake manifold gasket leak which can then cause damage to the engine.
However if it were coolant the mixture should look sort of green and you'd be losing coolant in the process.
As long as you're not losing coolant and it's just moisture in the engine then it can be normal and nothing to worry about to have moisture in the engine oil.
My truck gets moisture in the oil when I don't drive long distances and I was concerned myself and took it to a mechanic and they said it was just natural moisture that will do no harm to the engine.
When the humidity is high that can happen a lot.