Even if it is brown or nearly brown. Once in an exposed to light area within the water it will bounce back.
Co2 is not needed to grow but it is needed forthe intense pearling that you see in other tanks.
Quickest way to get it back up and running would be to detach it from the slate and let it float.
Then it will be closest to the light and bounce back quickly.
Any parts that do die off can be removed and you will most likely have twice as much after a week or 2.
Andy