I am sure
@Chow_13 is spot on.
It seems to be an spider mite infection, so you have to be really carefull, as it can spread easily to other plants AND if not cleaning correctly, they can survive till your next grow.
For me, i use B.A.C - Vitality +, as foliar spray, once or twice during each grow, but i haver my doubts if you can get rid of them by just a shower for the plants.
But i am pretty sure its spider mites.