Me personally think the new sand, even with old water and rock is basically a "new setup". So the system hasn't reached equilibrium yet, ie; nitrogen cycle not in balanced yet with the bacteria that's already in the rock, also not yet in the sand and not in the balanced concentrations (how's that for a run on sentence)
. So while it's reaching equilibrium, any excess extra nutrient will be feeding the hair algae. The increased MH lights also is a variable that's helping enable photosynthesis in the algae to take up the nutrients. Decreasing photo period will help...
It'll slowly come back to balance, but it doesn't hurt to get a Sea Hare to keep the growth down while everything is coming into balance.