Soaking Driftwood

[razorkai]

So I have this large piece of driftwood I got from my lfs. Predictably it floats so I have been soaking it for a couple of weeks in a plastic dustbin outside. I checked on it today, and apart from still floating it seems to be developing some sort of slime like algae on it despite being covered with a lid in total darkness. I am obviously going to need to soak it for a while to get it to sink but I don't want to end up with a horrible slimy bit of green wood. What's going on, how can this stuff grow with no light? Any suggestions as to what it is and how to get rid of it?

Thanks.