I don't believe that GAC causes MHLLE, but some may say that it takes certain trace elements out of the tank, or the dust can cause an irritation. I think a poor diet and poor water quality play more of a role in MHLLE. These are just my opinions though.
I did have a cherub angel, suceptible also, develop MHLLE as a seconday infection due to ICH. It came through with a wide variety of healty foods and I feel that I keep my tank is pretty clean.
And to answer your question about sps warfare, you can run carbon occasionally to aid in what you believe is sps warfare but I don't think SPS really release any toxins that you need to run carbon for. But I don't know for sure, they sometimes just get slimey.