Mold on hardwood in nursery?

I am nesting and setting up my nursery. We had a huge snowstorm about a week ago and I had a garbage bag from the nursery that I couldn't throw out because it was too dangerous to get to the cans outside. It's finally warm enough where the snow and ice have melted but when I removed the bag off the floor there was this huge black spot. We tried wiping it down then scrubbing it then my SO took some sandpaper and tried to sand it out but it's still there. It wasn't there last week. Is it possible it's mold? How can I remove it? I'm really worried because it's where the crib it going to go and I want to make sure it's a safe environment for my baby.