As far as the proximity of the shrooms
to the leather.....Think how many times in one hour your filter and power heads "turns" your tank. If its a reef, your suppose to turn it over no less than 10 times in one hour. If you have a 55 gallon tank that your "turning" 10 times an hour, you are moving 13,200 gallons every day "through" the tank. Its not really going to matter where you put the corals...if there is toxins in the water, they will have 13,200 chances to come into contact with the corals in the tank.
Now..the good news. The toxins are very easy to remove. Activated Carbon....Dont listen to the people that tell you that its only good for 48hours. I used to believe them until I did my own research. Your going to get anywhere from a week to two weeks out of each batch of carbon once its in the tank. Put the carbon in a high flow area and let it do its thing.
Short story on this topic. I bought a leather coral about a month ago. In the store it was about 15-18 inches tall and had a crown just as wide. When I put it in my tank, my other corals closed up over night. A week later, they were still like that. It killed my toad stool and my finger leather. Obviously something had to be done..it was pretty obvious what was doing it since everything had been doing so well before the new arrival. Water change and a big batch of carbon. 2 days later, everything was back to normal and has been ever since.