Moisten the tile with clean water from your garden hose. Wet the area sufficiently but not too much to reduce the effectiveness of the scrub pad.
Gently scrub the area with a scrub brush to loosen all the dirt and other deposits present.
Rinse the area well.
Wet a nylon scrub pad with white vinegar.
Scrub the area and try to loosen and remove the buildup---in most mild cases, tan easy task.
Rinse well. If you have not removed the ring, move on to a stronger cleaning option.
Grout Haze Remover
Use a grout haze remover for cases of more moderate buildup. While designed to remove grout residue, this tool can be also help you rid your pool tile of the white ring.
Rinse the area well after scrubbing it with the grout haze remover. If you have not removed the ring, try a stronger cleaning option.
Take the edge of a putty knife and gently scrape the white ring off. If after a few gentle attempts it has not moved, then try a phosphoric acid--based grout cleaner.
Phosphoric Acid--Based Grout Cleaner
Wear gloves and follow the grout cleaner manufacturer's instructions.
Clean the tile with a scrub pad; you may have to apply a little pressure in order to get the white ring to come off.
Scrub with a brush and grout cleaner again, making sure that the majority of the cleaner gets onto the white ring.