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It's normal and harmless and will go away eventually on its own. You can scrap it off yourself too or maybe boil the wood a bit more but I wouldn't worry tremendously about it.
Oto's don't eat fungus. You can also remove the wood wash it off with water and pour hydrogen peroxide all over it. Let it sit 20 minutes and put back in the tank. Often times it won't come back after doing this.