This is exactly what happened in my case, before the green nightmare, i couldnt rid the water of the yellow, than green tint, it was very frustrating...I still to this day do no know what caused it just that algone cleared it all up ....I am leaning also towards the rocks although, many people said that in my situation and i dont think that was the case....
Try making your tank a simple as possible right now, i took everything out of my sump (media, rocks etc..) cleaned tubings, put a better skimmer on etc....run for 24/7 if you havent already....and water changes, it it keeps coming back even after the water changes than something is def not right....