That is probably superficial gelcoat cracks due to some approximate tightening of the beam straps. The strap we see seems to be overtightened (the bold twisted the strap on the left side). If the inside strap is not as tight the hull would move against the beam and generate this kind of failure.

Sand the cracked gelcoat over and if there is no crack in the polyester below simply cover with a new layer of gel coat. When you mount the beam after that add a small amount of silicone between the beam and the hull to help distribute the load better, and use a torque wrench to tighten everything equally.