It's hard to tell from the photo, but is your clone starting to flower?

If it's going into flower, it will stop growing and root development will slow down.

I wouldn't put the plant outside under direct sunlight at this stage. The plant isn't strong enough for that.

I would bring it inside and put it under some florescent lights on a 20/4 schedule to make sure it stays in veg until it gets strong and healthy enough to go outside.

You mentioned that the leaf tips were already going yellow before you got the plant, so there may have been a problem with over fert even before you got it.

Don't keep the soil too wet. Let it really dry out before you water it again. Let the roots stretch and grow and work for what it needs or else the roots will just stay and rot in the water soaked rockwool cube.