Looks like you have dinos all over the sand bed and rock. Dino's can be bothersome and even kill coral depending on the strain. Once you get the NO3 and PO4 test kits in post the results here. It could be a lack of nutrients in the tank causing these issues. I had a dino problem and it was due to my phosphates bottoming out when I added GFO to my system.
What is your feeding regimen for the BTA/fish? What filtration (mechanical, chemical, etc.) on the tank? Do you dose anything?
Stabilize what? Ca, alk, and mag, temp and salinity are all in normal range. Nitrate is a little high but not horrible. Phosphate is the same. Source water is from 4 stage RO. Running Rox carbon in a reactor. Honking big protein skimmer 13 inches in diameter and 2 feet tall that is rated for a tank at least 100 gallons bigger than my actual tank.
Wow that’s a lot of fish and must be an awesome sight to see, especially when feeding.
I still think the salinity swing and temp fluctuations that caused the BTA to be unhappy. Another possibility is a bacterial infection if a wild nem was introduced and it’s spread to the others.
Since you said a water change made it happy then go back to the basics. Just do a water change and only dose your elements. Nothing else. See if they react when you slowly add one thing at a time. And slowly meaning a week between dosing some new chemical.
Did the algae start when you stopped dosing Prodibio Biodigest? If yes then the bacteria is dying off and the nuisance is overtaking. Give it time and this bacteria strain will go away in a few months.
I find this happens when I stop any form of carbon dosing
Just my opinion, but I think the phosphates are pretty high. Typical want this in the .05-.10 range. Nitrate of 25 isn’t too bad. Water changes or media to reduce phosphate are options. You have a 250gallon? Media is probably easier.