The fact is large zoanthids (think Sunny D's, Margicians, pandoras) are often referred to in the hobby as 'palys' and, while it's convenient, its also taxonomically incorrect.
@encrustingacro has been persistently beating this drum for awhile for which I applaud them.
Now, bear in mind researchers are often reclassifying corals (and fish) but 'paly' specifically refers to corals like button polyps (texas trash), captain americas/jerks, and grandis palys. For anyone that's ever worked (ie fragged) these the difference is apparent. Paly tissue is thicker - denser- and weeps copious amounts of fluid (toxin) when the mat is cut.
The difference I explained above is an actual physiological attribute you can look at.
At this point we should all be aware things like feeding response, size and coloration are highly variable based on water parameters. That said, palys are more likely to display a feeding response with larger particulate (think brine and mysis) than say mid sized or smaller zoas (form follows function- smaller polyps are going to trap and eat smaller foods like phyto or say reef roids).
So, the term 'paly' has been broadly applied, incorrectly, to a number a number of larger polyped zoanthids and, as a matter of convience, has stuck.
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