The NASA movie shows a flowing liquid which forms under conditions near the Martian equator where water is stable. Dry ice (CO2) would sublimate to CO2 gas under these conditions. Pure water would likely not be stored in the near surface either because it would sublimate into water vapor. That leaves the possibility that the source of the water is brine or chemically adsorbed water. Hygroscopic (water loving) salts are thought to be the most likely way that water is stored by the authors of the study. The most common hygroscopic salt on earth is calcium chloride which is used to deice driveways and dehumidify closets.
The abstract in Science magazine says summarizes the discovery:
Water probably flowed across ancient Mars, but whether it ever exists as a liquid on the surface today remains debatable. Recurring slope lineae (RSL) are narrow (0.5 to 5 meters), relatively dark markings on steep (25° to 40°) slopes; repeat images from the Mars Reconnaissance Orbiter High Resolution Imaging Science Experiment show them to appear and incrementally grow during warm seasons and fade in cold seasons. They extend downslope from bedrock outcrops, often associated with small channels, and hundreds of them form in some rare locations. RSL appear and lengthen in the late southern spring and summer from 48°S to 32°S latitudes favoring equator-facing slopes, which are times and places with peak surface temperatures from ~250 to 300 kelvin. Liquid brines near the surface might explain this activity, but the exact mechanism and source of water are not understood.
I have a friend, Neil Coleman of the USNRC who published an article on the forms of the canyons on Mars. He gave solid evidence that they were the same forms as channels caused by flowing water on earth. Congrats to Neil. His work has been confirmed by this study.
This video raises the possibility of primitive brine loving life on Mars. Detailed geological and geochemical evidence shows that Mars once had far more flowing water early in it's life. This evidence shows that if life developed when Mars was warmer in wetter it may have been able to survive in subsurface brines because water has apparently been continuously present near the Martian surface.