@Wannikea ********
It does not matter how you bring in the money, maybe cash might be less traceable, but it is still your resonsibilty to mention you brought in the money.
I don't think tax authorities will be much after a few aliens bringing in some money, but when they want to do, and you did not tell you bring in any money, you stay over 180 days in the calendar year, and you are not supposed to work in the country, it might be difficult to explain how did you pay for cost of living.