This is a long-term, ongoing issue, yet I can't find anything eating my zoas. I used to have a 10" diameter rock completely covered in zoas, all of which disappeared over a very short period of time somewhere around a year ago. Since then, any frag of zoas I put on that rock also disappears. I thought it was the Astrea snails, as I'd see them attached to where dissipating colonies were (on frag plugs, etc). I since moved them to my refugium, yet zoas still disappearing. I don't have any fish that a) are not zoa friendly, or b) ever go near them.
I've read articles on zoo spiders, zoa-eating amphipods, etc. If there are predators, what is the best way to solve this crime scene?

As far as all the usual questions,
tank size: 300g w/ ~40-60g sump/refug
water parameters are fine (dkh/Ca+/Mg+/pH/Sal/Orp/K+/Fe)
nothing new introduced that would explain this (again, long term issue, across 2 tanks now)
SPS growing fine
LPS and softies fine
Only issue is with the zoas.