Looks sorta like a Pencil urchin or something. I could be wrong, but hopefully Cranberry comes thorugh here and lets you know for sure
and if it was a pencil urhcin I wouldnt think it was eating zoas. theres diets are different algeas, uneaten food. Prolly was just knocking the zoas around and banging them up. I hated having one in my 55 gal tank back int he day, but my gf thought it was kool before she got addicted to the hobby.
But maybe u have an URCHIN ZOANTHID PREDATOR!!!