Green Layer on Pool Floor (probably dead algae)

Hello all. I cannot get rid of fine layer of what i think is dead algae on inground pool. About 25k gallons. Have a sta rite 3 sand filter. Plaster/gunite, not really sure if there is any difference.

I have tried raising chlorine level to 10 ppm but not clearing. I haven't really held it there consecutive days, i think i did hold it at 6 ppm two days in a row. I use liquid chlorine, 12%.

Pump has been on for a week 24/7 now and not letting up. I will add some DE periodically. The pump gauge does go up to 14, 15 pressure and I back wash. Green crap comes out. I keep filtering but that stuff still there.

I tried vacuuming to waste. But my system don't have vacuum to waste setting. So i just set it to backwash and manually vacuum. I did that yesterday but this morning floor still the same. After vacuuming, I raised chlorine to 12ppm last night and this morning still about the same chlorine. Floor still same.

I had water tested and was told combined chlorine is about 2.5ppm and i needed to throw in all 4 gallons of chlorine to shock pool. I told lady already at 12ppm and she said its not the ppm, its about how many gallons you throw in???

I have 4 lbs of potassium chloride oxidizer i plan on throwing in tonight. Any ideas, suggestions??Also, plan on changing sand. Its about 9 years old.

Additional info, sorry forgot. CYA is about 25 to 30. I know i neeed to raise it but wanted to clear this stuff first.
Alk is 100
Hardness is 200
Ph before adding chlorine was 7.4-7.6 range. Today is above 8 probly due to high chlorine.
Filter gauge reads 9 psi on fresh backwash. Right now its about 10 psi.
I use the Taylor 2006 test kit.

If the color is green, it may not be dead algae. Usually the dead stuff is brownish. Your algae is probably just continuing to reproduce. Try raising you chlorine to 20 or more for a day or two. Then, brush, brush, brush, and backwash often. I always try to keep my CYA around 50 at the start of the season. It seems to help the chlorine.