Oh. Well, the nudibranch may or may not eat them, but you can easily kill them with any of a number of solutions.
Kalkwasser mixed into a thick paste squirted all over the anemone.
Lemon juice injected into the anemone
some have had luck with injecting boiling water
I have killed them by injecting alkalinity solution (soda ash)
There is a product on the market called "aptasia x" that seems to work well.
Even after all of these are dead, if you stay in the hobby for a while, you'll definitely see more here and there. I would manually kill them and then keep a treatment on hand for future applications.