I was in my basement today, which is unfinished. The exposed wood is covered with a pastic sheet. I noticed on some of the boards there is a black film. Is this mold? It has been humid where I live and maybe the plastic over the wood created a mold growth situation? If this is mold, do I simply scrap it off, or should I treat it with something? Thanks, Joe.

More than likely it is mold/mildew. The plastic is keeping moisture in and not letting the joist breath.
Why is there plastic over the joist? Dirt floor?

You can clean it off with a diluted bleach solution.

The basement is poured concrete. There was plastic all the way around the joists in the basement when I moved it. I just thought is was for a protection. Sounds like I should tear that plastic down. Thanks for the help, Joe.

You should remove that plastic from the joists. I'd also try to vent the basement or add a dehumidifier. You can just scrap off mold. You can make a 1:3 mix of bleach and water or buy a professional mold/mildew spray. Spray liberally and allow to air dry.