A healthy Naso tang may eat up the Dictyota. Emerald crabs can sometimes eat it too. In my opinion, I'd save the diadema aka long-spined urchin (that's the only one that eats Dictyota) for a last resort as it also eats corraline algae, and eventually it will clean off the live rock until it's white. IMO, white live rock isn't very natural looking or even appealing.