Your cactus needs to be watered when the soil is dry. Water it real good and pour the water out of the tray so it will not sit in water. Too much water will make it store up too much water and give it root rot. It needs at least 5 hours of sun a day and if it is in a window, it needs to be turned around to keep it from leaning toward the light.

The top part depends on the bottom part for its nutrition. It can not live alone.

It needs less water in the winter.