We had those growing up and unfortuanetly had to dump the water at the end of the day. Mosiquitos yes, mold grows, bugs etc. It was GROSS. Not to mention today, people are concerned about child drownings which can happen in any amount of water.
We always dumped the water then stood the pool up to dry int the garage.
I would not feel safe keeping even a small pool of water on my property, again for the child safety issue. Health issues 2nd.