How to clean concrete pool coping?

Maybe it is mineral build up? If so straight vinegar on a cloth laid on the spot for a few minutes and the rubbed should help.

Inspect the entire length of coping for damage. Check for loosened or broken pieces of coping as well as chips in the stone or concrete. Look for loosened caulk along the outer edge where the coping meets the pool deck. Lack of caulk can cause water damage.

Dip a Mr. Clean Magic Eraser into the pool and wipe stained areas vigorously. Magic Erasers remove light scuff marks and smaller stains. This is best type of cleaner for the curved plastic coping of an in-ground, vinyl lined pool.

Clean heavily soiled areas with an acid-based coping cleaner. Apply the product according to the instructions and scrub with a soft brush to avoid scratching the coping. You may need to apply the product and allow it to sit for 10 to 20 minutes to increase effectiveness. Specialized coping cleaners are best for tile and ornate coping. Test the product on an inconspicuous area before beginning on the main coping areas.

Mix a solution of 20 percent household bleach with water in a spray bottle. Apply to the stained areas and allow the solution to work into the coping. This bleach solution works well to remove stains caused by tannins.

Use a power washer to direct a high-velocity stream of water at the stained areas. Before you start, make sure the coping isn't broken, missing caulk or damaged. You won't need cleaning solution to remove stains using this method. Be sure to wear safety glasses or goggles.

Yes, I believe its mineral buildup plus the sun has made it dull. I will definately the natural less harsh solvents life vinegar & magic eraser before trying the bleach or the chemicals. The whole deck needs power washing but I've heard it's not good to do until cement deck is 3-4 years cured. Thank you!

Ditto on magic eraser

Vinegar can do the trick!