GHA is a wonderful addition to an aquarium, cough cough. On a serious note, as someone mentioned, the live rock could have been saturated with phosphate. How we used to do it in the "old" days, late 90 early 00s was exactly what Cpage did. We'd receive live rock, submerse it, skim the water and change out the water completely, once a week. I think a lot of places do not do that anymore (not cost effective to the retailer). If you disinfect the tank, just use vinegar or buy a bag of lemons, cut each in half and rub everything down that route.