Never bleach driftwood is correct - tanks yes; when they dry out the bleach is gone but driftwood will hold it deep within and release it long after. If a plant is growing on the wood I see no simple solution. You could try a chemical treatment designed for algae in aquariums but do it in another (fishless) tank and then after treatment is successful (week or so), change the water, run charcoal (change twice) for a week and then put it in the new aquarium. Guess that tank would need a light.

Me, I'd clean it by hand and then just do lots of water changes to prevent the algae growth.

Lights being on 12 hours a day is rather long - maybe the intensity is too low?