My guess is that pool store was assumed because of the mention of Phos-Free. This is a product that is widely pushed by pool stores - as far as phosphate removers go it's quite expensive and removes relatively little phosphate. Please don't be upset, we see Phos-Free pushed by pool stores all the time.

Post #3 is puzzling, If you used only dichlor and trichlor I'd expect CYA to be much higher. Did you change water this summer? Is it possible the pool went for some time with little or no FC?

What kind of algaecide have you used?

Mustard algae usually appears as yellow patches on the bottom or sides of the pool (in shady areas) that disperse readily with a brush. As it grows the yellow will become greenish. It usually doesn't cloud the pool. Phosphate level 125ppb or lower helps suppress mustard algae effectively.