  Aims. The aim of this paper is to consider relationship between the decay of
sunspots and convection via the motion of the family of granules and how the
diffusion mechanism of magnetic field operates in a decaying sunspot. Methods.
We report the decay of a sunspot observed by the 1.6m Goode Solar Telescope
(GST) with the TiO Broadband Filter Imager (BFI) and the Near-InfraRed Imaging
Spectropolarimeter (NIRIS). The analysis was aided by the Helioseismic and
Magnetic Imager (HMI) on board the Solar Dynamic Observatory (SDO). In the
first step, we followed the decay of the sunspot with HMI data over three days
by constructing its evolving area and total magnetic flux. In the second step,
the high spatial and temporal resolution of the GST instruments allowed us to
analyze the causes of the decay of the sunspot. Afterward, we followed the
emergence of granules in the moat region around the sunspot over six hours. The
evolution of the trees of fragmenting granules (TFGs) was derived based on
their relationship with the horizontal surface flows. Results. We find that the
area and total magnetic flux display an exponential decrease over the course of
the sunspot decay. We identified 22 moving magnetic features (MMFs) in the
moats of pores, which is a signature of sunspot decay through diffusion. We
note that the MMFs were constrained to follow the borders of TFGs during their
journey away from the sunspot. Conclusions. The TFGs and their development
contribute to the diffusion of the magnetic field outside the sunspot. The
conclusion of our analysis shows the important role of the TFGs in sunspot
decay. Finally, the the family of granules evacuates the magnetic field.
