It's actually a white fungus that grows due to some nutrients that are still in the DW. Boiling the DW prior helps prevent or reduce this, but overall it's a temporary occurrence and should subside once the fungus has depleted the food supply your DW is leaching. You can manually remove it if it bothers you, but it won't harm anything to leave it. When this happen to me in the past, my otos added it to their menu.