I was referring to my earlier comment to take a turkey baster to gather up a sample of the water with the algae in it and squirt it onto a white paper towel, then smear the grains and report back the color and texture of the smear. It helps distinguish mustard algae from sand or dirt or pollen, etc.
Just so you know, we can't answer the question of why until we know whether you're dealing with mustard algae.
Normally, in a TFP-method pool with a fixed/known cya, a passed slam, adequate circulation, and FC brought daily to the upper end of the [fc/cya][/FC/cya] range and without dropping below the min., you simply would not see algae (unless it was already established, in which case it multiplies faster than dosing of FC can kill it.)
Regular algae is a sure sign of under-chlorination and a 7.5% ratio between FC/cya usually keeps it in check...meaning, prevents an outbreak.
So that it didn't in this case to me must mean either that there was algae hidden behind your light niche during the slam, that the FC level wasn't stable at a min. of 7.5% daily without dropping, that its not algae at all but pollen/dirt/silt from your filter, or that its mustard algae, which is chlorine resistant and requires special disinfection measures.
Just so you know, we can't answer the question of why until we know whether you're dealing with mustard algae.
Normally, in a TFP-method pool with a fixed/known cya, a passed slam, adequate circulation, and FC brought daily to the upper end of the [fc/cya][/FC/cya] range and without dropping below the min., you simply would not see algae (unless it was already established, in which case it multiplies faster than dosing of FC can kill it.)
Regular algae is a sure sign of under-chlorination and a 7.5% ratio between FC/cya usually keeps it in check...meaning, prevents an outbreak.
So that it didn't in this case to me must mean either that there was algae hidden behind your light niche during the slam, that the FC level wasn't stable at a min. of 7.5% daily without dropping, that its not algae at all but pollen/dirt/silt from your filter, or that its mustard algae, which is chlorine resistant and requires special disinfection measures.