If it is a white fuzzy growth on the wood it is bacterial, and bleach will fix it, along with a good brushing from a wire brush. THIS IS NOT AN ALGAE. Bleaching wood will not 'poison' the wood, but will kill any bacteria near the surface of the wood. Chlorine will evaporate when left out of the water in only a few hours. If you want to avoid this in the future, just boil the wood, bleach it, or get Malaysian driftwood.