Some nudibranchs are reef-safe, other are not. Most have very specialized diets that may be "impossible" to replicate in our closed systems. Some have toxins in their tissues that can damage your tank when(if) they die. Most nudibranchs aren't suitable for closed systems at all. Having said that, I've heard of and seen "Algae Eating Nudibranchs" as well as Lettuce Nudis which in this case were different looking specimens. Some people have success with Lettuce Nudis, but I don't know much about the ones I saw that were simply labelled "Algae Eating Nudibranchs" (and weren't Lettuce).
Either way, many snails eat diatoms. Try a couple Astraea snails.
