but sinces my roomate brings those mini cactus from cameron highland
It is mainly do to the type of soil mixture you have. It should be about 80 to 90% sandy medium. It's best to have them in clay pots as clay pots let the roots breathe and wicks the moisture out keeping the roots from rotting. Plastic pots keep the soil wetter and the soil can't breathe. Water your plants sparingly about twice a month. Make sure it can drain and keep it in in a bright to sunny location.