It looks like magnesium. But maybe you have the wrong PH, because of this block of elements.
Do you have a PH meter?
https://www.wietforum.nl/uploads/monthly_2018_10/marijuana-deficiency-chart-jorge-cervantes.thumb.jpg.43e1bbec1925b6fc1e203a4807c496d5.jpg
And as colleagues below advised. It is better to rinse the earth / coconut, adjust the PH. And only then water with fertilizers.