Paint on a deck will never work. Pressure treated wood will not accept paint for long.
Sanding as suggest on a molded up deck will grind the mold deeper into the wood. Far better to use bleach and a pressure washer to kill the mold first, let it dry out, then sand.
Here's the rule a new pressure treated deck needs time to dry out before staining. In you area it could have been 6 months. If you try to stain it to soon it will fail every time and the mold will be sealed behind what every you put over it and keep coiming back.
Solid color stains can be mixed any color that paints can be tinted. The first time it was sealed it should have had two coats, if not it's never going to last.