Near the end of the ep, she was seen harvesting some hidden plant- or fungal life, presumably for food. Aside from that, NASA-dude presumably had supplies leftover from his mission (enough to feed four dudes for at least a year, presumably), and his subterranean home near the magical heatsource may well have had other edibles in it.
There’s also the question of how long the planet’s been like that. If it was really a desolation caused by the creepy dust-ghost, it could have come at almost any point, so theoretically not-so-very-long-ago (in cosmological terms), it may have been life-bearing, thus explaining the pond-monster, et. all.
Or, ya know, a blue gel melted onto their only dust-proof camera so they just said fuck it and went with no sun for the eps filmed on that soundstage/in that locale ;).