Actually the anemone doesn't have to come into contact with the leather for there to be an issue. Leathers, as well as most soft corals, excrete toxins as a method of defense and to claim their space on the reef. In the ocean they are dissipated quickly but in our tanks they can build up and become toxic to some species. With as many cabbage leathers as you have it is very important to run ample carbon to remove the toxins as well as doing frequent water changes. Do a search on Soft Coral Toxins or Leather Coral Toxins for more information. Here is a link I found in a quick search. http://www.ultimatereef.net/forums/showthread.php?t=120934 - http://www.ultimatereef.net/forums/showthread.php?t=120934  There is plenty more infor available on this subject.

 

If you aren't running carbon the level of toxins may have been sufficient to affect the anemone before you ever added the first one. Here is what I would do before trying another anemone.

 

1. Put a pound of carbon in a media reactor, canister filter or even just a filter bag placed in a high flow section of the sump. The more water that can pass through the carbon the better. Run this for 30 days.

 

2. Do a large water change, at least 25% and replace the carbon. After 1 week add the anemone.

 

3. You can probably cut back to 1/2 pound of carbon changed monthly after that.

This can be the case sometimes, there are many different strains of cyanobacteria most of them metabolize different things. However in most cases test for nitrates will come back low due to the fact that the cyanobacteria is consuming it as it is produced.

Sorry to hear you're now battling Cyano. I had a similar issue last year. The red slime started building up almost immediately after cleaning the tank. I tried blasting it with a turkey baster, reducing feeding and light cycles, and most of the other suggestions in the above posts. Nothing seemed to help. In my case the red slime looked like a stringy mess and had some bubbles mixed in with it when I allowed it to sit for a couple days without touching it. I ultimately lost several SPS corals and was ready to give up. 

I was in the process of moving so the tanks weren't getting as much attention as they should. I eventually tracked down the issue to a white eggcrate that I was using in my frag tank both as racks for corals and to build a plenum. I removed the white eggcrate frag racks and within a few days saw a marked improvement. Apparently some of the white eggcrate can leach phosphates into the system. My experience is that cyano is more tied to phosphate than nitrate and that the water levels will test close to 0 phosphate because the cyano is eating it up as fast as it is produced. 

Shortly after this incident I had to move and set up a new system with about 120 gallons of water volume. The new frag system is built from stock tanks because of their large volume and low height. I have a 50 gallon bare bottom frag tank, a 50 gallon refugium with 4 to 6" DSB over 70% of the bottom, a 20 gallon skimmer section and a 28 gallon Nano all plumbed together. Things were going fine for a couple months and then the red slime started to appear again in the refugium. This time I added Cheato and Caulerpa to the refugium and saw no decrease in the cyano.

I had been looking into bio pellets to increase the processing of nitrate and phosphates. I got a Two Little Fishies Phosban reactor and some biopellets form Bulk Reef Supply. I set up the reactor and within a couple weeks there was no sign of cyano and it hasn't been back since. Everything is doing great now. One thing I did in addition to adding the biopellets was to add some RN Bacteria. It was suggested that the addition of bacteria to the system would speed up the colonization of the reactor and I had some RN Bacteria (used for the Hiatt System) on hand so I added it to my system. 

Overall I am very happy with the biopellets and it may be worth a try in your system. You don't have to dose a bacteria supplement as the pellets will colonize with bacteria on their own but the literature on BRS suggests that it will take 4 to 8 weeks for the filter to become established and dosing bacteria like ZeoBack can speed up the process.

Good luck with your battle. I know from experience that this can be frustrating.

Oh also with regard to the engineer goby, If he is constantly stirring a majority of the sand bed it may prevent the sand bed from doing its job properly. This is because anaerobic bacteria found deep in the sand bed cannot become established. This bacteria is responsible for processing nitrate to nitrogen and keeping nitrates low. 

Probably fine, I would change carbon after the leather sheds JUST to be sure.  

Doubly so if its a big piece.