Usually some kind of silicate. I like Calcium silicate, but potassium silicate is good too. I find its best to not overcomplicate the res. Less is better imho.
Get your temp and humidity up further if you can. Like 75-78f and 60-65RH.
The coco is far too dry also, it should never lighten up in color like that. Need to keep coco wet, don't let them dry out, and dont feed plain water ever, try to keep the ph and ppm consistent each feed. And feed several...
Then letter rip and see what happens, sublimation would dry it out eventually but it could take a long time in the absence of a vacuum. Just have to be carefull not to move them around, they would be prone to losing trichs frozen like that.
What is the "seed raising" component of your "perlite, peat and seed raising blend"?
Looks like Ca deficiency, maybe potassium. Why though... not sure.
What is the water source? Are you testing for ph and ec/ppm or are you just raw doggin it with no testing? Eyeballing the nutes etc...