I had exactly the same problem with a new boat I had. ( the settlement papers prevent me naming the brand)
I was told it was mould wax, but the normal cleaners did nothing.
They sent me some cleaners from the manufacturer and they did nothing.
started to break down with the antiskid breaking up.
It was literally crumbling in my hands.
Finally my lawyer (pit bull) got the attention of the dealer in OZ after arresting one of their boats.
They tried a few things without much luck.
Ultimately they offered to have a new deck
shipped out and they would install it on the hard
. (I don't think I have ever laughed so hard)
Finally after a year in court I got a new boat. Even to the end they fought us as our model had been discontinued, they tried to give us the model 3 ft smaller saying that was the equivalent even though the brochure said the one that was one foot bigger was the development of mine.
They also argued that mine was now second hand!
Sold the new boat within 24 hours of it arriving in OZ and have never gone back.
In the process I did experiment
(sssshhhh) with a few products. None of the fiberglass
cleaners worked. The only thing that worked was a product for bathroom cleaning
called "easy off bam". I don't think they make the one I used anymore, but it contained oxacylic acid.
It really worked a treat, took it off clean and nice. Some heavier staining required two application.
1. Wear gloves
2. Wear gloves
3. Did I mention wear gloves
4. Hose the area down with a LOT of water
5. You will need to polish the area once done to get it sealed.
By the time we had discovered this the deck was a mess. We were referred to as "the s#%t boat" on Wendesday nights and it wasn' t because of my sailing abilities I don't think??????
One other thing, it will turn brown over time from UV exposure.
Sorry I couldn't be more positive. It was one of the most horrible experiences of my boat owning life. Mainly because of the dealers attitude. Took me 3 years to even consider buying
Good luck with it