I have noticed on random islands that I can walk over shoreline rocks to be met with the poisoned effects. And no matter how hard I try and check, I cannot see even one part of the Starfish.
And due to this being on console, antidote is 2 Pipi and therefore much more valuable to the player, until it is farmed.
Variety of procedural islands.
Default, no custom maps or settings.
Random rocks have poison starfish clipping them.
Unnoticeable, they poison you without warning.
The only workaround after you have confirmed the location of the purple starfish is to build a base floor over it to then stop any further poisoning.