Hey, what's happening?
I am by no means an expert here but I just had the same problem. It's called for a lack of better terms "Red Slime". It looks like red hair covering the rocks and sand right?
Anyways, just go to your local fish store and let them know that you have red slime growing in your tank and they will give you something to take care of it. I just added some to my tank the other day and I can already see a difference.
If that doesn't work, hopefully Mike "the expert" will see this post.

Good luck