Looks like a harmless fungus that normally occurs on heated wood-my shrimp, snails, pleco. ottos love feeding on it and it resolves on its own with time, however, if you don't have anything in the tank to help eat it-you may want to remove on occasion and scrub it off in some cool dechlorinated water. It can some times cause your water to cloud some and water only change should clear it up
How big is the tank-filter, live plants-did the wood sink okay
Be careful using vinegar on thing like wood that can absorb it-more due to the pH change it can cause- also bleach can be absorbed and cause problems for fish....be careful.....good that you didn't use it.......