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It's probably a type of algae; I had stuff like that growing on my driftwood in my 56g. It didn't seem harmful --the corys nibbled at it a bit, and the snails I put in there from my 10g have made short work of clearing most of it off.
If you have that much leftover food, you're feeding your fish too much. Remember that their stomaches are no bigger than their eye, so only give them as much food as they can eat in a couple of minutes. Once your tank is cycled, you'll want to add a 'clean-up' crew to take care of the food that hits the bottom. Depending on the tank size, that could be a snail or two, some corys, or regular catfish. When you're ready to add them, read the relevant sections of this site about the clean-up crew you want to add, to make sure they don't get too big for your tank (some catfish can grow to a foot or more).
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