If you want them to go away, there are many ways. Nudibranch's, certain types of damsels, chemicals, chemistry etc....choose your potion.
Salifert has a product called Flatworm Exit that customers have found very effective.
Nudi's will be effective, but die soon after.
Alkalinity raised and maintained can also help.
I think there's also some advice about them in Julian Sprungs book Algae Secrets.
Good luck, they can be pesty, but harmless in many occassions.