I have some branches I sanded down and used from outside.. I didnt really do much to them other then sand them nice and smooth. I have read that you can put them in the oven for 30mins on like 300d maybe even 250 depending on the size of the branches. I think the issue your having is that the piece has mold in bedded inside the wood or piece you have. My frogs are amazing when something like this happens they will stay away from that piece in the tank so I know to go in and check it out. I have had some issues with mold so I know how you feel. If I think of anything else I will get back to you on it.