I think they're just a little under nourished. Slight purple on stem and leaves just a little pale suggest that to me, together with general skinnyness.
Are the roots being constricted from growing out of the rockwool?
Check the roots, what do they look like?
I can't see any fan, you need a fan blowing on the stems to fatten them up.
Other than that, it must be too little light, I think.