Another Green Pool

Just arrived back from 6 week vacation and it appears the pool (which was being 'overseen' by a neighbour) salt chlorinator appears to have died a few days after I left, and short of 'throwing a few litres' of liquid chlorine in, neighbour has done basically nothing. The pool was swamp green, and probably in that condition for weeks.
After 3 consecutive days of Slamming, pool water is restored to clear - however large areas of the floor now have green algae 'stained' into the pebblecrete surface. No amount of scrubbing with pool brush on 2meter telescopic pole (pool depth 1.5meters) is budging the 'stain' - it's embedded. Even heavy hand scrubbing with stiff bristle brush in shallow swimout area has little effect.
All the Slam guides quote shock dose chlorine, scrub, vaccuum, backwash, shock, scrub, etc, etc - however the chlorine isn't making any impression on lifting/killing the embedded algae on the floor.
I did dose with 5litres of algecide alongside the last chlorine dose, but it hasn't had any impact on lifting the stain.
Short of draining and pressure cleaning, is there any way forward to removing stain (any stronger chemical treatment?)

75,000litre pebblecrete, salt water chlorinator
CYA 30, Slamming to 15ppm twice daily for 3 days
Water now clear but algae stain showing no sign of lifting

Attached pretty ordinary photos
 

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Just arrived back from 6 week vacation and it appears the pool (which was being 'overseen' by a neighbour) salt chlorinator appears to have died a few days after I left, and short of 'throwing a few litres' of liquid chlorine in, neighbour has done basically nothing. The pool was swamp green, and probably in that condition for weeks.
After 3 consecutive days of Slamming, pool water is restored to clear - however large areas of the floor now have green algae 'stained' into the pebblecrete surface. No amount of scrubbing with pool brush on 2meter telescopic pole (pool depth 1.5meters) is budging the 'stain' - it's embedded. Even heavy hand scrubbing with stiff bristle brush in shallow swimout area has little effect.
All the Slam guides quote shock dose chlorine, scrub, vaccuum, backwash, shock, scrub, etc, etc - however the chlorine isn't making any impression on lifting/killing the embedded algae on the floor.
I did dose with 5litres of algecide alongside the last chlorine dose, but it hasn't had any impact on lifting the stain.
Short of draining and pressure cleaning, is there any way forward to removing stain (any stronger chemical treatment?)

75,000litre pebblecrete, salt water chlorinator
CYA 30, Slamming to 15ppm twice daily for 3 days
Water now clear but algae stain showing no sign of lifting

Attached pretty ordinary photos
3 days of SLAM isn’t likely to clear a green pool. You probably just need to keep going a bit more time. Can you post up water test numbers?
 
What he said ☝️☝️
Also if you can get in more doses that may be helpful.
SLAM means- Shock Level And Maintain
The Maintain is important.
Algeacides don’t do what you think they do (they are a preventative at best) & many contain copper which can further stain your pool. Copper can only be removed through water exchange.

You can test the stains to determine if they are organic(algae) or metal stains. If they are metal stains you can deal with them after the
SLAM Process is completed.
 
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