I personally am skeptical of the idea that sunscreens are actually responsible for marine ecosystem changes, rather than climate change or other large forces
I would've said so, too, before my GF got into the reef tank hobby. Corals are ridiculously sensitive to pretty much everything. Light, chemicals, nutrients, water flow, diurnal rhythms, you name it. It's fairly believable that something in the low parts per billion might affect them adversely, particularly organisms that are already stressed.