Worley noise
Noise made from the distance to the nearest scattered point, drawing cells like pebbles or foam.
See it in Noise & texture. Set the noise type to Worley.
In plain words
Scatter some points over a surface at random. For every pixel, measure the distance to the closest point. Colour the pixel by that distance, and cells appear around each point. The result looks like pebbles, cracked mud, foam or animal cells. It is also called cellular noise.