Above is an enlarged section of the OP's photo. Notice the relative uniform shape of the dark areas and how the white gelcoat fades away to the dark area. Also notice the white streaks in some of the dark area. The dark areas are all in the valleys.
Thus far, no one has reported success at removing these kinds of dark spots, because it is not dirt, it not mold, is the fiberglass under the gelcoat. Gelcoat is sprayed into the mold when the deck is built and it is not an entirely uniform thickness, thus, some areas under the nonskid are a little thinner or thicker than other areas. After 30 years of scrubbing, the thin layers wear out. The gelcoat is thinnest under the valleys in the non-skid.
There is no easy fix, all will require standing the non-skid smooth and adding new nonskid. There are options that range from gluing on a rubberized non skid, a one part fix like Kiwi Grip, painting and adding grit like Soft Sand, or using gelcoat and
Gibco Flex mold. The other option is just learning to live with it as it is.
Wish there was a more optimistic solution. One thing for certain, more aggressive scrubbing will only make it worse.