Mold in rock wool ?

It could be mold, but also looks like calcium buildup from hard water. If your plant does not seem to be affected, I'd just watch it. If your plant seems stressed, I'd be doing some cleaning. I grow in rockwool and have experience similar and it was just hard water.

This is SPARTA! Get that out of the grow space. Clean all your equipment, learn from the mistake and mive forward. You don’t want any of that mold in your space, or in your body with the end result. ❤️👊🇨🇦

Ciao! Mmm la muffa non va bene, attento che non sia partita dall'interno, dalle radici! Mi sembra infatti proprio quella muffa che parte dai marciumi radicali.. Non ho mai usato il metodo ebb&flow, preferisco dwc oppure drip system :) poi attento perché mantenere la lana di roccia "scoperta" andrà a rovinare le radici quando saranno alla luce! Devono rimanere al buio.