Are they already rooted or are you in that stage now? If trying to get roots the MH light might be a touch much atm. The yellowing could be the clones feeding off of themselves. I would cut back on how often the rockwool is being watered, mist the clones and put a dome on them. If you dont cut back on the rockwool being watered and dome them the rockwool will stay way to wet and rooting will either take forever or worst case they perish from to much water. I just went through this on my last grow and I'm glad I caught it in time but slowed down the whole process by about a week and a half.