I have had this with wood iv collected myself, I believe its the sap being released from the wood then going bad in the water, I usually get around it by boiling the wood or putting it in the oven for a wile (dont burn it), this should remove any saps or bad stuff on the wood.
This. Happens once when moulds feed on the leftover sugars in the wood. After the first time it shouldn't happen againFungus, it's pretty common when you put relatively fresh wood into a tank. It's harmless and will disappear after a few weeks.