Bleach may or may not be cheaper than liquid chlorine. You'll just have to check prices in your area. But, it is readily available; you can buy it everywhere. Most of us just use the generic Walmart bleach. The concern with lots of cal-hypo granular shock is it can cause cloudy water problems and also when the level gets too high, there is no way to reduce it other than to partially drain your pool. You don't have either of those concerns with bleach.

Yes. You need to maintain your chlorine level at 15 or you will never get rid of the algae. Take a look at the Best Guess chart in my signature below.