That's really hard to tell, I think the white stuff on the rockwool is nutrient salts, but if there is some on the main stem, then it could be fungus. If you can gently scrape some off of the rockwool, and inspect it you may be able to identify it. If it's a dry, crystalline powder, then it's salts.
Some organic nutes also include some organisms that grow on surfaces. This stuff is for the beneficial critters to eat, so don't jump to any quick conclusions.
if u think it is mold get a sulphur based spray and treat the affected area. that will inhibit the mold from growing by changing the PH of the surface.
so i've for sure got something growing on my rockwool. it's a DARK shade of green and doesn't seem to be interfering with their growth, but i'm worried it's gonna get worse...should i be worried?
yea i got those black/white shade things, they just don't cover enough sometimes so it's still growing on a few of em, nothing major to worry about though right? just try and shade em better?
Algae isn't harmful by itself but the problem it poses is that it eats the same food as your plants. So as long as you've got algae growing you've got something eating nutrients out of your reservoir that isn't part of your harvest.
Plus, there's some bugs that will eat algae that will also harm your plants, so it's better to just eradicate it as best you can. You can kill it a number of ways, but the best is to simply block it from light. If that doesn't get rid of it all, a dose of H2O2 can help.