Green Again

The kids have been swimming A LOT in this HOT weather. The water level was down about 4-5 inches. Yesterday evening I checked the FC - 3.6. I added bleach and added treated well water from inside house (carbon filter, iron filter, de-ionizing water softener). I checked the FC last night after I raised the water level back up and the FC was 6.4. I woke up this morning and the water was green. Not bright green, but definitely green.
I checked the CC and it was nonexistent. So:

- What causes water to be green? Is it Algae only or are there other causes?
- How often do you have to add water to your pool? I just added water last month and it turned green then too. But I let the FC get down to almost 0. This time there was definitely enough FC in the pool at all times.
- Should I bring the FC up to 15 to get rid of the green?

In hindsight I probably should have ran the filter all night.