First, it is not a cactus. It is Euphorbia polygona 'snowflake'.

It grows well in full sun, or part shade, but not too much shade. Water when dry. It tolerates cold down to a few degrees below freezing, and given adequate drainage, can stay outside year-round.

They don't need much care, just find a good spot, and leave them alone. These are S. African plants that are quite at home in California latitudes. Thirty-five degrees South near the ocean has climate quite similar to 35 degrees North near the ocean. If you can, plant them in the ground. Your plant(s) look like an six+ years old clump. They have either male, or female cyathia (flowers), but can sometimes do a midlife sex-change. ;)

Although it has no spines, like all Euphorbias, it has a irritating/toxic sap that seems to come out with the slightest brush. Be careful when handling this plant. Do not brush your eyes with the back of a contaminated hand or glove. Wash thoroughly with soap and water after contact.

Good rescue. Happy growing!

Brad