I have two fish and inverts in the new tank.ohh gotcha. Does the new tank have livestock? If it does not, using the new rock and a course of salifert flatworm exit would be wise and would likely take care of any that made it thru.
Let the old infested rock dry for a week or two and it should be safe to use again, or you could also treat it with FW exit.
If I do start seeing them again what are my other options? With the nudibranch you mentioned why do they have to be removed once they eat the worms ?