You could try feeding seaweed, algae pellets/wafers or something else. You could also potentially try "snail-o" which is just something like gelatin + powderized algae (something like Repashy's Soilent Green). Then there's the tried and true method of growing algae in the display tank, and then transferring that to the QT. You could do this by adding something like coral frag disks/tiles and waiting for algae to grow on them, and then transferring them as needed into the QT (make sure to bleach them after they've had the algae eaten off to make sure that no nasties get into the tanks!)