Sea fans have structures known as sclerites.
These are calcium carbonate deposits that help make up the skeleton. When the fungus invades the coral tissue, the coral produces a higher percentage of purple sclerites, thus purpling the tissue. We think this is a protective response that sea fans have to limit the spread of the fungus. A cure is not on the horizon. Once we are satisfied that we know the origin, spread and pathogenesis of the infection, we can think about how to interfere with the overall process.