Critter Consuming Euphyllia

[NanJ]

I thought I'd throw this out for ideas...
"Something" is eating a Euphyllia here, a Euphyllia there. It ate over half the heads of a hammer, consumed all three heads of an Octospawn. This week it has eaten over 90% of a beautiful green Alveopora whose skeleton is the size of a tennis ball.

The rampage happens at night. My husband stays up past when the tank lights go out and says the fish are asleep so he doesn't think it is a fish. Since we've been noticing the damage, he has been specifically watching the Alveopora and never sees any suspects, day or night, so we assume a critter is coming out in total darkness after he goes to bed.

Our 300g tank is 5 years old so there may be a peppermint shrimp lurking somewhere but we never see it if it is there. Years ago we had a tiger tail sea cucumber but have not seen it in probably 4 years. There is a tiger shrimp that we see maybe once a month. Those are the only "hidden" critters we can think of that might still be in the tank.

Based on how quickly the Alveopora has been eaten we think it has to be a critter of some size. Anyone have ideas or similar experiences?

 

[ISpeakForTheSeas]

Any pics of the corals?

My first thought would be Euphyllia Eating Flatworms.

 

[Cell]

What and when was the last addition?

 

[NanJ]

I did not know of Euphyllia flat worms! Yikes. We immediately dipped (Coral Rx and Reef Dip) the pitifully bare Alveopora but I did not see any flat worms come off. I also dipped another Alvie that has not been as extended as it used to be, again no flatworms. With lighted magnifying headpiece I examined for eggs and flatworms but didn't see anything suspicious.

The last coral addition, a Cyphastrea, was this summer but this Alveopora has been in the tank for probably 3 years or longer.

One attached photo is the Alveopora after the dip. Just a few days ago all those places on the skeleton were polyps. All that are left are the few in the upper left.
The other photo is what came off when I dipped.

 

[Uncle99]

The speed of death reminds me of attack by fish,rather than crab, shrimp or slug.
What fish you have in there.

 

[Cell]

How long ago did you first notice damage? I doubt it's euphyllia eating flatworms if the last coral you added was a cyphastrea months ago.

 

[NanJ]

The speed of death reminds me of attack by fish,rather than crab, shrimp or slug.
What fish you have in there.

I thought fish, too, but my husband observes the tank a lot and says he never sees any suspicious fish activity. Plus, the damage happens after the lights go out and the fish are mostly asleep.
Tangs (Naso, Powder Brown, Tomini, Purple)
Foxface
Skunk Tile, 2 Diamond Gobies, Royal Gramma, 2 Filefish, 3 Firefish, Canary Blenny
We've had this group of fish for at least 2 years with no coral eating up until now. I know their appetites can change, though.

How long ago did you first notice damage? I doubt it's euphyllia eating flatworms if the last coral you added was a cyphastrea months ago.

We first noticed issues about 6 weeks ago. Good point about timing.

 

[Derrick0580]

My guess is the powder brown. Powder blues are notorious coral munchers. Buddy of mine just rehomed his due to this exact problem. Corals all stayed irritated and retracted but couldn’t pin point any issue as his parameters were great, just happened to see the pb taking a bite by chance one day.