"how do I get the black mildew color off the"
There is no "black mildew color". That stuff IS THE MILDEW FUNGUS/SPORES and you need to remove it all, either chemically or mechanically. Bleach, steam cleaning, hydrogen peroxide, 3M Scrubbing pads....
And if all that fails, you entomb it in epoxy paints.
After all the cleaning is done, remember that the mildew WILL COME BACK unless you take steps to make sure the boat is kept warmer, drier, and well ventilated at all times.