Great photos - those egg capsules are all open (see the little open circle in each egg capsule - that is the 'trapdoor' that opens at hatching) and I can't tell whether they hatched before or after the Potassium Chloride treatment. At hatching the AEFWs probably swim straight into the coral skeleton (but some may spend some time swimming around in the tank). If these eggs hatched before the treatment the hatchlings in the coral skeleton might have had some protection from the increased salinity from the corals own slime, alternatively like the adult worms they might have been killed. Hatchlings are microscopic (0.3mm) so you would need a microscope to assess the impact of KCL on them. If they hatched after the treatment it means that the Potassium Chloride soln wasn't effective at killing off the embryos in their egg capsules.
Your findings are really encouraging especially given the resilience of the Acropora to the treatment, and they have given us some very focused questions to answer.
1) can hatchlings and juveniles (that you can't see with the naked eye) survive this treatment?
2) can the Potassium Chloride solution kill off embryos in the egg capsules?
Flatworms, like corals, use mucous as a protection against environmental changes. Your KCL treatment seemed to work well against the adults and we can now look at the effects on the younger stages.
We are restocking our AEFW tanks with new Acropora now (after a die off due to bacteria) and we can try these experiments shortly.
I think AEFW can grow quite quickly, so if you didn't repeat this treatment for a while, say 4-6 weeks, then did it again and saw small AEFW (5mm) fly off then that might tell you that eggs and hatchlings can survive the treatment.