How do I politely notify my neighbor....

[Giesela]

If it were me I'd figure out how to keep the cats from the balcony. <shrug>

While this doesn't upset me, I have been upset about things that don't upset anyone else so I understand your feelings.

But as a coworker used to repeated me remind "circle of control"

You can control the cats getting up to your balcony. Harder to control cats or your neighbor. Exert control where you can and let the rest roll.