A few years ago I had a lump of Mopani that went far more furry than the usual. Picked the brain of a fishy friend of mine who suggested adding a 100g bag of Fluval Clearmax to the filter or to hang it in the aquarium somewhere close to the offending wood.
It worked...not sure how, why or if it was actually anything to do with the Clearmax for sure but the fur went after a few days and never returned
So in my cupboard of "fishy things to try when all else fails", I have sachets of Clearmax waiting to be used when needed.