The wood has started growing a layer of what could be either bacteria or a fungus, which is apparently common in aquariums with a higher pH. I have seen this happen on a re-used piece of mopani wood before: it usually stops growing the fluff after a while. Without analysing a sample of the fluff, it would be impossible to find out if it really is a bacteria or a fungus. I took the wood out of the aquarium and washed it off. The fluff feels slimy to the touch, but is harmless.