Opening our pool this spring

Millipede - they live in mulch, leaf litter, even grass thatch. Primarily eat decaying vegetative matter. They need damp areas. Do what you can to remove nearby mulch, grass piles, leaves, any decomposing vegetation, and keep the area dry, or at least not damp all the time.
Increase in numbers may be seasonal - with more in a wet spring, and few in a hot/dry summer.
Completely harmless.

Next up, likely will be earthworms in the pool after every rain......
 
I have them too. Know as millipedes around here! Our local pest control guy sprays for them every spring which helps for a while. Says they live under concrete.