Well...
Nudibranchs can be hard to ID, so based on your description it may be hard to tell if it actually is one. It may be some sort of sea slug or something else to which the term 'Nudibranch' is commonly attached.
eg: this is a flatworm:
this is a nudibranch I think:
'true' Nudibranchs are all carnivorous and usually have very targeted diets on *specific* sponges or other animals. For the most part, we dont know what they eat until we observe them in the wild feeding - and if we do know what they eat - odds are vary good you cannot get those specific animals for home aquaria.
Is you have a sea slug or something that may be herbivorous or an obnivorous grazer, then perhaps you will get lucky and have something that it can eat. There are a few species that are well known that are grazers such as the lettuce nudibranch (which
is not a nudibranch but a sea slug)
In any case... the life span of most of these critters is a year...2 at the most. So if you have an adult it may not have long to live anyway, even if you can manage to feed it. In general, nudibranchs are animals that should neither be collected, sold or purchased. Best thing you can do is take it back.
good luck, here is some more info if you want to read a bit more:
http://www.animalnetwork.com/fish2/aqfm/1999/may/wb/default.asp
Steve R.
[ January 29, 2002: Message edited by: Steve Richardson ]</p>
Nudibranchs can be hard to ID, so based on your description it may be hard to tell if it actually is one. It may be some sort of sea slug or something else to which the term 'Nudibranch' is commonly attached.
eg: this is a flatworm:

this is a nudibranch I think:

'true' Nudibranchs are all carnivorous and usually have very targeted diets on *specific* sponges or other animals. For the most part, we dont know what they eat until we observe them in the wild feeding - and if we do know what they eat - odds are vary good you cannot get those specific animals for home aquaria.
Is you have a sea slug or something that may be herbivorous or an obnivorous grazer, then perhaps you will get lucky and have something that it can eat. There are a few species that are well known that are grazers such as the lettuce nudibranch (which

In any case... the life span of most of these critters is a year...2 at the most. So if you have an adult it may not have long to live anyway, even if you can manage to feed it. In general, nudibranchs are animals that should neither be collected, sold or purchased. Best thing you can do is take it back.
good luck, here is some more info if you want to read a bit more:
http://www.animalnetwork.com/fish2/aqfm/1999/may/wb/default.asp
Steve R.
[ January 29, 2002: Message edited by: Steve Richardson ]</p>