Yes, a deep clean wouldn't hurt. This point it take patience for your filter to filter out all of the dead algae. Have you tried using DE? This should speed up the final phase of your SLAM. Pool School - Add DE to a Sand Filter
Make sure that you are not backwashing until your pressure has risen 25% above the clean pressure. Sand filter work best when they are a little dirty, so backwashing too soon is not good.
When adding DE, try to it right after you have backwashed. You want to add just enough DE to raise your pressure by 1 PSI. Try 1/2 cup at first as you don't want to do too much.
Add the DE after every backwash.
Take photos at the same time each day to help you track your progress. Photos of the stairs (if they are in the pool) are helpful in measuring clarity improvement.
Make sure that you are not backwashing until your pressure has risen 25% above the clean pressure. Sand filter work best when they are a little dirty, so backwashing too soon is not good.
When adding DE, try to it right after you have backwashed. You want to add just enough DE to raise your pressure by 1 PSI. Try 1/2 cup at first as you don't want to do too much.
Add the DE after every backwash.
Take photos at the same time each day to help you track your progress. Photos of the stairs (if they are in the pool) are helpful in measuring clarity improvement.