Thanks for the great info and advice!

But I still can't figure out why the problems only occur if the pump is running (whether on Filter or Recirculate). If there is no algae when the pump is not running, where could it be? It would have to be in the pipes since it makes no difference if the water is being filtered or not. But where in the pipes? What could hold enough volume of algae to cover the entire pool in 12 hours?

I would think that I had some breach in the pipe(s) that was causing this. But as stated, I had the pipes pressure-checked and the guys said no breach.

Any additional help would be appreciated.