It's new tank syndrome. GHA is part of the cycle. You will deal with cyano next. Just keep cleaning the rocks and sand bed. Then do large water changes weekly for a few months. If large water changes don't keep no3 and po4 down you could add a reactor with roaphos or some other GFO or phosphate RX. It will slow down in time without the nutrients in the water. Also consider keeping the white light % low for a while.