We had the same problem with a Boat we owned
It is mould releasing wax which turns yellow with exposure to UV.
The more intense your UV, the faster it turns yellow.
In my experience, you can polish all you like, it won't get rid of it. It simply hides the problem.
The only solution we found was oxacylic acid, followed by a stiff polish. Ours was in the non skid and smooth. We ended up in court when Builder
washed there hands of it (our deck
actually started to crumble).
Good luck, I hope you have a better experience than we did.