I had pretty good luck removing a minor dictyota infestation (~3" sq) by just scrubbing the rock with a toothbrush. I had to do it three times, but the dictyota is gone now. I'd take the rock out and scrub it as soon as I noticed new lobes sprouting. I also coated that section of rock in kalk paste the last time I scrubbed it, but I'm not sure if that helped get rid of it or not.