Algae in itself is not dangerous to swimmers. However,if algae is growing that means the chlorine levels are not high enough to kill the algae-- and if they're not killing algae, they're also not high enough to kill other stuff in the water, like bacteria and viruses. If you could post a set of water test results, it would help us diagnose the problem better. Also, what size pump and filter do you have? Do you see anything blowing out of the returns when the pump is on?