Leathers release toxins for a couple of reasons. One is when they are stressed or are getting chemical signals from similar or other toxic corals close by. The second reason is that they shed, and in the process of shedding they release a compound that also contains the toxins. If your tank has other leathers and softes only in it, it should be at to much of a risk. If you have other corals such as sps and lps then depending on the species they can have trouble dealing with it and it can lead to death. If you run a mixed tank then its always good to run carbon, good skimmer and other forms of filtration to deal with it, also more frequent water changes are also a help.
Mojo