I’m pretty sure the white stuff that appeared on the ground is frost. Looking at it from this angle and other angles you can see the frost slowly forming and then melting again when the water hits it.
I think this is best explained by the intense shock waves decompressing the air, which causes it to cool down and take heat from the ground (of course they also compress the air by a lot, but with shock waves the time interval that the air is below atmospheric pressure can be much longer than the time interval that it is above atmospheric pressure). interestingly, there are also thermoacoustic refrigerators that function in a similar way.
Edit: I now think that cold gases from engine startup are more likely to have caused this, because as the static fire just started in the video you can see spots freezing over as those gas clouds approach them.
Taken from the overhead drone shot https://twitter.com/SpaceX/status/1695159316670718231
as threelonmuseteers said, as long as the gas does some kind of work while expanding (which could just be exerting a force on the surrounding air), it will expend energy which will cause it to cool down.
@m_rln @rafadavidc @spacex I muted that account because it had taken over my feed due to posts being ‘boosted’ by SpaceX