Zoatastophe

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.

Most common issue...
Nudis
Spiders
Asterina stars
Fungus
Zoa pox
Predator fish
Leaching of into tank



And can you post your actual parameters?
Any pics of the frags that look unhappy at the moment?



Solving issues

Pods- Mandarins,wrasses etc
Nudis and spiders- dipping and late night watching spiders hide inside of the zoa you won't seem them easily. The nudis have a glow to them with all blues on or a uv light.

Dips for zoas
Lugols iodine
Melafix
Revive
Coral rx
Etc.

+1 to above. Id add, wild crabs,(grorilla and others), gall crabs. So a crab trap.
Also slightly possible chemical warfare. A coral in the tank secretes an enzyme that suppresses others growth. Gorgs and leathers are famous for it. You'd want purigen or carbon in a reactor.

With zoas disappearing in the sump the same way, id go with chem war, or Like CMD suggested something leaching in they dont like.

Thank you both for the information. Sorry for the late reply, but needed to further my own investigation.

I ended up trying a Lugol's dip, and discovered dozens of whatever the hell this thing is. I haven't been able to find any information. Looking for identification and eradication assistance, if anyone has insight.

The picture is magnified considerably. The actual size of these things (this is a larger one) is about 1-2 mm. The dark spot is the tail end. This is not a nudibranch. It seems to have more of an exoskeleton body structure than a nudibranch, and is far too fast-moving.

https://www.dropbox.com/s/1iffvynxhv..._Pest.jpg?dl=0

Incidentally, found this great site for identification. Unfortunately no leads here, but could help others:

https://www.lionfishlair.com/hitchhikers-guide/