A lot of sunscreens have the ingredient avobenzene which when mixed with the chemicals in a pool or minerals in well water will leave behind stains that are nearly impossible to remove.
Do not try to treat stains with bleach, it will only discolor the items more, instead, try waterless hand cleaner. The alcohol in the cleaner will help to break up the stain, while the gel makes it easy to get it where you need to go.
Let it sit for an hour or so and then launder as you normally would.
Speaking of laundering, if you suspect hard water is the culprit, add a cup of baking soda to each wash load to soften the water and neutralize the chemicals.
If none of these seem to be doing the trick, wet a plain bar of soap and scrub it over the stains as a pretreater. The soap bonds with the sunscreen to help remove it.
Easy ways to remove sunscreen from clothes! I'm Mrs. Fixit and it's just that simple!