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This white fur is fungus, and it will grow on any uneaten food on or above the substrate. Reduce this by feeding less (whenever uneaten food remains and develops fungus, you can be certain that too much is being fed for the fish). Deal with any leftover if it does occur, either be removing it or having critters that will deal with this.
Substrate fish usually prevent this, but make sure you acquire fish you like and that fit in with the current inhabitants and tank size. One should never acquire any fish solely to deal with some "problem."
Snails are a valuable resource for this. They will usually ensure nothing remains, and by ingesting food and waste they break it down into smaller particles that the bacteria can then more easily handle.