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It is fungus. It's not uncommon for it to appear on DW. You can give it a spot treatment of Excel or Hydrogen Peroxide which often times kills it off. You can take a needless syringe, pull up 2-3ml of Excel or Peroxide per 10 gallons of tank water, put the syringe close to the fungus and squirt it slowly. If you use Peroxide you need to turn your filter and lights off for 15-20 minutes during treatment.
Use 3% Hydrogen Peroxide, what most people have in their medicine cabinet. For a 6 gallon tank you can only use 3ml tops of peroxide once a day. It's not much in a tank that small.