Nudibranch resembles a neon colored slug and can either be smooth or have lots of equally colorful tentacles. Nudibranch's have undergone detorsion, where their mouth and anus for a long continuous tube. Torsion is the process that shelled gastropods go through to rotate their mantle and shell 180 degrees to bring it directly above their head. Due to going through detorsion, Nudibranch's are bilaterally symmetrical. They have very simple eyes which are typically limited to sensing light and dark. They breathe through a naked grill that forms branchial plumes in a rosette in their back, where the name Nudibranch likely originated from.
They are hermaphoditic (containing both sexes) but cannot fertilize themselves. Hermophoditicism is a common trait in sea animals to increase likely hood of finding a mate. Think to a Nudibranch every other Nudibranch you meet is a potential partner, instead of being limited to one sex or the other.
So the big question remains, why did Nudibranch's develop such bright colors? well it seems Nudibranch's took two alternative approaches, one was to blend in the other to stand out. Some Nudibranch developed colors and appendages to mimic plants around them, an extreme camouflage. While other Nudibranch took the opposite approach and developed bright colors to stand out and warn predators that they are poisonous or distasteful. Some Nudibranch feed on hydroids then store the hydroids' nematocysts (stinging cells), they can travel down their body with out harming the Nudibranch. They likely do this with a large vacuole surrounding the nematocyst. There are some Nudibranch's that can produce toxins on their own without dietary aid, there is even a type of Nudibranch that can excrete acid out their skin.
To sum up, the Nudibranch is more than a colorful sea slug, it is a highly specialized creature showing every color of the rainbow. That's it for now,