  We study the dynamics of clusters of Active Brownian Disks generated by
Motility-Induced Phase Separation, by applying an algorithm that we devised to
track cluster trajectories. We identify an aggregation mechanism that goes
beyond Ostwald ripening but also yields $z=3$. Active clusters of mass $M$
self-propel with enhanced diffusivity $D\sim$ Pe$^2/\sqrt{M}$. Their fast
motion drives aggregation into large fractal structures, which are patchworks
of diverse hexatic orders, and coexist with regular, orientationally uniform,
smaller ones. To bring out the impact of activity, we perform a comparative
study of a passive system that evidences major differences with the active
case.
