Welcome to the Bunnings Workshop community @Tanya11. It's wonderful that you've joined us, and many thanks for your question about repairing cracks in Pebblecrete.
Are you looking for an aesthetical repair to the Pebblecrete, or does this crack continue into the pool and become structural?
Any repair to this crack that I can think of could potentially end up looking worse than the current appearance. Typically, the crack would be chased out and opened up with an angle grinder, epoxy glue would be poured into it, and then you'd sprinkle matching pebbles over the epoxy. I suspect it would still be quite noticeable, though. However, even a repair with a Pebblecrete mix would still be apparent, as it will look new versus the old Pebblecrete surrounding it.
I look forward to hearing more about the issue and providing assistance.