Difficult to see in your photo, but if they are on top of the water and generally cling together, they could be fleas or small ants. We had them several years ago. Storms and very dry weather often drives them from grassy areas. They are attracted by the water’s reflection. Otherwise no idea.
Use of a strong pool shock will kill them off -- then vacuum up. But you should also treat around your surrounding yard with appropriate garden poisons that won't harm barefoot children or pets, if they are able to have access to the area. Best to do it when pool is not in use and children/pets can be kept inside.
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