This is a 2D diffusion-limited aggregation (DLA) model of frost crystal growth. A handful of fixed seed points act as nucleation sites. Vapor "walkers" spawn at the edge of the domain and take a random walk — a tiny random step every frame, mimicking the erratic Brownian motion of a water-vapor molecule bouncing around in cold air.
The instant a walker drifts within sticking distance of any already-frozen particle, it freezes in place and becomes part of the crystal; a new walker immediately respawns to replace it. Because a walker sticks to whatever it touches first, the tips of the growing crystal intercept incoming walkers before particles deeper inside ever get a chance — this is exactly why DLA growth is fractal and branching rather than a smooth, solid disk, and it is the same statistical mechanism believed to shape real frost, snowflakes and mineral dendrites.
Raise growth speed to run more random-walk steps per frame, add more seeds to start several crystals competing for the same vapor, and turn up branchiness to make each walker's steps larger and jitterier, which produces looser, spikier arms instead of tight, dense growth.