They can have very long lasting pelagic larvae or benthic miniatures of the adults.
I used to remove egg masses when i found them and sit them in a 5 gallon tank with an air bubbler and some byropsis. I would check daily, and soon I would have some tiny nudibranchs.
the last time I tried it they were pelagic larvae so I got no where.
If your larvae are pelagic, probably be better for you to just sell those and get more. IMO