Could be mildew,but the mildew i had was never that bad so it could be something else.Is it really humid in your room?You could buy mildew mold killer at your hydroponic store,ask them what is the best stuff to kill it.But i would get a confirmation to what it is first.Good luck.
^^^^
what he said
And I think it's a mold of some sort. Separate it from the rest. Raise your temps. Which both will suck more moisture out of the air and make a hostile environment for mold to grow.