White Build-Up in Main Drain

[tidnab]

Hello. New pool owner. Currently working on SLAM and running into some issues with holding the chlorine overnight. Decided to investigate all the cracks and crevices. Opened up the light niche and found a large build-up of white $h'*. Opened up the main drain on the bottom and found underneath was some algea build up, rust, and another large build-up of white material. I can chip it off with a screwdriver so I was down there doing that laat night. No idea what it is but was wondering if anyone else knew? Wasn't sure if it was just plaster from the original build or some buildup of something? Tried to get the best picture can, I can swim down and get a better pic if needed. Was wondering if anyone thinks this is what has been eating my chlorine?
Also did a quick lick and it just taste like a sandy rock, for anyone curious.
CH: 475
TA: 60
PH: 7.2? Middle of SLAM
FC: 16
CYA: 40
Salt: 3800 ppm

 

[Britali]

Ack no clue! Hopefully, someone here can help.

 

[Britali]

I do wonder if could be plaster dust that collected in there & did not go through filter. And does look like it would give algae some nooks & crannies to hide in. Algae will eat your chlorine up so hopefully as you kill it, should be easier to maintain overnight.