I had 2 lettuce nudi's but now just one, I had to keep them away from filter intake a lot which is where the second one probably ended up. any way, had to look up the one you described would love to see a pic! this is what I found, by the way I don't know much about it but unless someone else kinows more and says it should be taken out if it looks cool, and is not harming anything I say leave it be.
They are carnivores that slowly ply their range grazing on algae, sponges, anemones, corals, barnacles, and even other nudibranchs. To identify prey, they have two highly sensitive tentacles, called rhinophores, located on top of their heads. Nudibranchs derive their coloring from the food they eat, which helps in camouflage, and some even retain the foul-tasting poisons of their prey and secrete them as a defense against predators.
Nudibranchs are simultaneous hermaphrodites, and can mate with any other mature member of their species. Their lifespan varies widely, with some living less than a month, and others living up to one year.
