I’m not sure what you mean by “balcony capping”? is this the balustrade or roof? Anyway, speaking generally I am not sure if you can do anything about this, under the Strata laws, at any rate. You might have by-laws that apply, so you should check those carefully. But if you have the default by-laws, I doubt if any would apply to this situation. And in reality, there is always some risk of objects falling from balconies. While pot plants on the top of the balustrade might increase the risk, whether this is actionable depends on several factors – if the balcony and capping are strata property or owners property, on the design of the balustrade and balcony, if other residents are doing or have ever done something similar, and what is below that area, for example.
I would suggest you raise the issue at the next strata committee meeting – ask your strata manager or strata secretary to add an item on the agenda about the problem, and see what the discussion results in. Also, in that agenda item, ask that the residents be put on notice in writing that they are responsible should any damage or injury results from a pot from their apartment falling on something or someone below.