Agree with yellow and six line. Green coris wrasse also great, but gets much larger than the yellow. If the infestation is severe.....lots of adults seen with blue (night) light viewing or flashlight then my suggesting is break-off and pull all montis from tank. Otherwise there may be a chance for natural predation. I've tried both ways, in the end the nudis won and I went without montis of any kind for almost a year. Keep in mind, my montis were attached to rock and simply dipping was not an option.
This is not an easy battle!