The volume of water being drawn from and the volume of evaporation, precipitation and runoff affecting that body of water, dwarf this desalination plant by an exceedingly long way.
It's not the overall volume of slightly saltier water, it's the hot, concentrated brine that's damaging. Most systems are designed today such that the brine is only ~50% more salty than incoming water and only a few degrees warmer, but it's still a significant ecological issue.