I had that problem with some driftwood. Everywhere I look it just says that it's some sort of bacteria feeding on something that's in the wood (sap or something). Eventually it goes away on its own once the bacteria's food source is used up. You can take the wood out and scrub it and boil it but it will probably come back. Basically all you can do is to wait it out.
A forum community dedicated to aquatic plant owners and enthusiasts. Come join the discussion about collections, displays, tanks, styles, troubleshooting, reviews, accessories, classifieds, and more!