Waxes, sunscreen oils, and other surface contaminates can start showing yellowish tint over time especially in warm, sunny climates.

To remedy this issue many boat owners have found success using Xylene which can be purchased at most hardware stores.
Next, using gloves and eye protection soak a rag with a generous amount of Xylene and then lay the rag on the discolored area for a few minutes. Next, scrub the yellow spots using a handheld deck brush (soft to medium bristles) on the affected area.

If the discoloration remains, repeat the previous steps.

Finally, rinse the deck thoroughly with soap and water.