brushing it with soap and water will remove it. If you have mollies or shrimps, or algae loving fishes, that should be gone.
Soap is BAD
. Don't do that!
I would suggest giving it a H2O2 dip and then brushing it while it's in the dip. You can probably do a 4:1 water:H2O2 ratio and be fine as long as no plants are attached to the wood. If there are plants attached to the wood, pull them off the wood and do the dip.
Plants are sensitive to H2O2 and some will melt if kept too long in the mixture