Classic Secondary crater swarm
A cluster of material ejected from one impact makes a sub-orbital trajectory and splashes down some way round the planet. Sometimes these are linear patches and merge into rays, other times splotches like this.
With such a short travel distance, a cluster of loosely-associated debris doesn't split up much, even after being blasted out of the atmosphere and re-entering, or sometimes it's a sign of partial break-up of a single fragment during this process.
Given enough time and effort one can map out their distribution and usually identify the primary crater responsible.