I have a few pieces of driftwood in my 75g tank, but this one piece that i picked upped @ lfs has been in for around 4 months now. This piece of wood is always growing a white cotton film over it. I take it out and scrub it down like every 2 weeks then in a couple of days the cotton material is back. It's an eye sore to me. How can i get rid of the growth and do you think my fish are @ risk?