this happened to me, with some wood i bought at my lfs. but i put them in my tank and found out that aquarium safe dose not always mean exactly what it says. anyway the wood leaked tanic acid(this also turns the water brown) and molded for about a week then died off after awhile and i pulled it out during a water change and use my tank water and a soft bristle brush to remove the mold and put it back in the tank and was pretty much gone after that all it takes it time and it will eventually go away. if you have not put it in your tank yet dont just leave it in a bucket of water till its done molding and such then add it this is the best way imo. i say that because i have tried boiling and bleaching and has not worked for me yet.
hope this help good luck
hope this help good luck