it's normal. happens to every single piece of wood i put in my tank. i think it has something to do with the tannins not being able to oxidize. it's fine.
I boiled the heck out of my spider wood then soaked for days. I got most of the tannins out but it still did what you're seeing when I added to the tank. I learned that it's just the organics still in the wood coming out. Mine went away in about two weeks. My fish, shrimp and snail loooooved picking at it. (it's just awful looking though)