I found the culprit. Pulled this gnarly, segmented worm with sort of a bobbit like mouth (not a bobbit, though) out of a sand tube randomly while moving some rocks around. Not a bristle worm, I think it's some kind of https://en.wikipedia.org/wiki/Chaetopteridae . A good 2-3 inches long, and thick like a big earthworm.
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I exposed it for a while and my leopard wrasse absolutely ZAPPED it. Gobbled it right up and spit out the head.