It looks just like what I've been battling for the last two months, but I've got it beaten back. I've run GFO in a reactor, cut back on feeding (especially dry food), done big water changes, two 3 day blackouts, increased flow, blown it off with a turkey baster every day and sucked up half of my thin layer of sand trying to get it under control.
It started to recede when I started dosing 2 part to get my alkalinity and calcium in line.
It's probably a combination of everything that is helping, though. That stuff is a bear to get rid of!