certainly could be.....algea on your rockwool which, amongst other things can affect how much oxygen the roots are getting and that your ph being off could lead to the problem you are seeing.

Off the top of my head....during bloom I'll use 25ml of Cal Mag per 2 and 1/2 to 3 gallons. Are you using RO water? What is the ph of your system at? That's number one.....if you aren't keep tracking of your ph, then that is where you need to start.

I would consider humidity far too high for flowering plants at 60%. I haven't seen fungus or mold issues in plants in real life but it certainly would be something I'd think about here. Or both.....or.....even more.... Anything above about 45% rh will leave your plants more vulnerable to mold.....I aim for the 30% with rh.

Let me find that link.....brb.....