One fish reported to eat the Montipora nudibranch is the Radiant Wrasse (Halichoeres iridis). The one in the picture is similar in some ways to the Monti-nudie do if you can get an H. iridis, then you may get lucky. Most Halichoeres will give you some flat worm protection as well.

Sixlines are supposed to be good for control of pyrmidellae snails that are not secluded inside tunicates but they do not eat nudibranches or sea slugs.