Blue Velvet Nudibranchs are sensitive, and don't acclimate very well, so don't be surprised if it dies before it eats any. the advice to up the flow is great advice, I forgot to mention it.
I have had 2 for 4 months now.
One large and the other about 1/2 the largers size
Experiences vary I suppose.
Maybe chalk it up to JL having exceptional livestock.