I start seedlings & clones in 1inch rockwool.. I stay in 1inch cubes until the roots are busting out the cube, then I place into larger cubes (4inch in my case)
As mentioned above, when you soak your cubes, be sure to shake out excess water..I like to grab the cube with both of my hands, and pretend like you're going to spike it into the floor a couple of times. Try not to crush or deform the cube
Are your cubes staying wet for long periods of time? Could this be causing your PH to fluctuate?
You don't want your cubes staying fully soaked for long periods of time without drying out.. This will invite all sorts of problems including slow growth, algae/green shit, & pests!
I feel like that 6x6 cube is way to big for that small seedling.. Were there any roots showing on the 1inch cube before you placed it into the larger cube? I wonder if the roots have penetrated the larger cube yet? I would think about removing the smaller cube (If the roots haven't moved into the larger cube), and allowing it to establish itself a little more before moving up to the larger cube
