Several choices on how to go with this. It is pretty common for wood to have something like this. Not harmful and fish often eat it. But if it bothered enough, you can pull it off until it stops, try treating it with peroxide or take the wood out to bleach soak it.
I'm guessing there is some form of sugar in the wood for a food supply but then there are likely to be thousands of these little things that we don't often see until we put them in the tank.
I tend to just ignore it and it leaves in a few weeks or so.