If I am not mistaken,it is actually a fungus that develops on a lot of new driftwood. It happened to all of the driftwood in my 90 planted. Once the nutrient supply is used up it should disappear.
The thing is that the particular piece of wood that this stuff keeps popping up on is not new.I have had and used it off and on for a few years now.I have tried scrubbing and allowing it to dry but it still comes back.I think it may depend on the species of wood?.....I try to ignore it.
I've only seen it on Mopani wood I've never seen it on "regular" driftwood. Do a search on Mopani wood there are several articals on the stuff. If Mopani is what you have.
I read before to cook it, then put it in water/see if it grows. If it does, cook it again, and keep repeating until it appears all the spores are now dead and won't grow anymore.
I've never had to do this though, as the pieces I had either were fine when I got them, or it went away real soon as nutrients probably dropped, and recently the cherry shrimp aided.