Thought the same thing as Stanzzz at first, the stuff you sometimes get on new wood in a tank (harmless), but apparently not. It's a matter of taste, but personally I like it.
I do woodworking, carved bowls and stuff, and collect a lot of downed wood, burls, etc. I leave it outside until I'm ready to do something with it and I find certain species of wood will grow their own types, colors of lichens, etc., some of them grow stuff that's unusual and pretty spectacular. For example box elder around here can get these very large, sort of trumpet-into-fan shaped, mushroom-like growths on it, hard to describe but pretty spectacular if you're into that sort of thing.
--The relevance I'm suggesting is that maybe you have a less common species of driftwood so it's growing an algae (or something similar) you don't typically see in a tank.