Assuming the pool service was not using Trichlor pucks, they could visit once a week and add chlorine by having the CYA be high so the chlorine lasts longer and start the week with around 14 ppm or so and may end up at 4 ppm or so a week later. However, in your case, it sounds like it got out of hand and the chlorine wasn't lasting well enough so algae was able to grow when the FC got too low.