Seems a little odd. A Watercress seeding apparently growing as an epiphyte (air plant) in a bottomless free-draining container suspended on a house, alongside an epiphytic cactus.
Perhaps it likes combination of good aeration (they don't like sitting in poorly aerated water) and high rainfall over winter. It found it's way up there from a tub below with watercress in it. The seedling is of course small, I'd be surprised if it reaches a useful size.
