Although manual removal of flatworms works, you may find it difficult (and tedious) if there are more than just a few.
For any corals that can be easily removed from the tank, I found that a 10-15 second dip in temperature matched ro/di water worked exceptionally well. Just gently swish the coral in the ro/di water and the FW's will drop like flies. The difference in osmotic pressure causes them to burst. The short exposure to the fresh water WILL NOT harm the coral at all.
I have also used Flat Worm Exit with limited success (2 times, at 2x the recommended strength)
What finally got them out of my tank for good was the addition of a pair of Blue Star Leopard Wrasses. I don't know the size of your tank, but ultimately, it was natural predation that did the job for me.
For the Nudibranchs - Get yourself a red LED flashlight and hunt them down after lights out. I would wait at least a couple hours after the lights are off, and leave the house lights off as well. Beware, also, that if there are Nudi's. there are eggs.