To update:

Friday: Backwashed filter and vacuumed during the day. Raised FC to 20ppm in the evening.

Saturday morning: water still cloudy. pH has risen to 7.8 - Plan on adding muriatic acid today to lower to 7.6, eventually to 7.4

Would adding 1/2 - 1 cup of DE in the skimmer when backwashing the sand filter help expedite the removal of dead algae?