What is it?
That looks like night blooming cereus, Hylocereus undatus, which I usually see growing up the trunks of palm trees. There are a number of night blooming cacti that are commonly called night blooming cereus though. It has high drought and salt tolerance plus, it can grow in a wide variety of soils. . They are easy to grow and don't need to be over watered since they are drought tolerant. They also need full sun for maximum flowering. Don't get disappointed if they don't flower for awhile as they have to get mature before the begin setting flowers!