With a CYA of 80, keeping your chlorine levels in the 20s is nowhere near high enough to kill mustard algae. See the "best guess chlorine chart" linked in my sig for more information about where your chlorine levels need to be. If you still have brownish spots on the bottom, then it's not dead yet. You need to raise the chlorine according to the chart, and hold it until those brownish spots are gone. You do also need to soak all the stuff that was in the pool before putting it back, so you don't add the algae back to the pool. You can put it all in the pool while you're holding shock level if you want, and kill both of those birds with one stone.