Not sure if that is Mopani or not, but I had some of that gunk on a piece of Mopani I had in an older tank, and I think it is some kind of fungus. I took out the wood, soaked it in some vinegar and then boiled it, and that got rid of it. There's maybe an easier method, but that worked for me.
"The art of being wise is the art of knowing what to overlook." -William James
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