The only harm from algae is when it appears on live plant leaves and increases thereon; this can smother the leaf and eventually kill the plant. But algae on wood or rock is normal, and often beautiful, creating a more natural appearance. I have a lot of this algae on bits of wood in some tanks. I just keep an eye on it and if I see it spreading to live plant leaves I then consider why this may be happening (it is due always to an imbalance of light/nutrients) and take steps to adjust things.
Wood is particularly prone to algae, since the wood is organic and thus a high source of nutrients. Algae will therefore readily take hold on wood.
I just took a photo of a portion of my 40g to illustrate...you can see a thick growth of the fluffy form of brush algae in the branches but nothing on the plants. This has been like this since June.
Byron.
