Algae blooms are a perfectly normal part of swimming. I swear every other time I take my kid to swim lessons the pool is green and they're just running a local community pool. These big time Olympic venues pretty much have unlimited resources to make sure little Johnny Croatia isn't going to go to the hospital every time he does a half pike twist.
In fact I hear this is a plan on the part of Rio, every day the pool is going to be the color of one of the Olympic Rings. Next up Yellow!