Barry,
Does the swim platform have the rough finish to it? That is the non-skid surface. You could try and acid wash it, or using a wax remover. If that works to your satisfaction and you go to wax the platform, do not wax the non-skid surface. It will probably look like it is oxidised again if you do. I am going to take a guess that what you see is from someone who waxed the area. I hope this helps.
Kerry
"I would rather be lost on the lake than found at home"