Tried boiling it a few times?
I agree, that's does not mean they are right though. a little research, on your part, will reveal my comments on the breakdown of fibre are true.Most people recommend boiling it to release the tannins...I did the same to mine....
Very likely to be fungus, you really do need to take it out and manually scrub it.
(I never recommend boiling the wood, for me much of the point is so it releases tannins into the aquarium!)