well if it is nudibranch they are spesific feeders and if they do not have what the eat in the tank they die. some eat algea and other corals and so on some eat atapia. So if they are nudibranch you will have to find out what they eat and move it out of your tank. Easiest and safest way to get rid of nudibranch's that is I guess. If it is flat worms read up on them on here there are quite afew post about it on here.

Bill